`cargo test` from workspace root no longer runs on MacOS

Start by reproducing the reported cargo test failure from the workspace root and the targeted cargo t -p opteadm and cargo t -p opte-ioctl commands. Inspect how these packages pull in libnet and platform-specific libraries; done means workspace-root tests no longer attempt to link unavailable illumos libraries on macOS.

More of an inconvenience than anything else, since cargo test invocations from lib/oxide-vpc and lib/opte function well enough. The problem packages appear to be opteadm and opte-ioctl:

kyle@Mac opte % cargo t -p opteadm
   Compiling bitflags v2.11.0
   Compiling serde v1.0.228
   Compiling ingot-macros v0.1.1
   Compiling opte v0.1.0 (/Users/kyle/gits/opte/lib/opte)
   Compiling memchr v2.8.0
   Compiling illumos-sys-hdrs v0.1.0 (/Users/kyle/gits/opte/crates/illumos-sys-hdrs)
   Compiling rustix v1.1.4
   Compiling cstr-argument v0.1.2
   Compiling nvpair v0.5.0 (https://github.com/jmesmon/rust-libzfs?branch=master#ecd5a922)
   Compiling postcard v1.1.3
   Compiling libnet v0.1.0 (https://github.com/oxidecomputer/netadm-sys?branch=main#e9bf1e51)
   Compiling terminal_size v0.4.4
   Compiling clap_builder v4.6.0
   Compiling ingot-types v0.1.2
   Compiling clap v4.6.0
   Compiling ingot v0.1.1
   Compiling opte-api v0.1.0 (/Users/kyle/gits/opte/crates/opte-api)
   Compiling oxide-vpc v0.1.0 (/Users/kyle/gits/opte/lib/oxide-vpc)
   Compiling opte-ioctl v0.1.0 (/Users/kyle/gits/opte/lib/opte-ioctl)
   Compiling opteadm v0.2.0 (/Users/kyle/gits/opte/bin/opteadm)
error: linking with `cc` failed: exit status: 1
  |
  = note:  "cc" "/Users/kyle/gits/opte/target/debug/deps/rustcuLHeB0/symbols.o" "<4 object files omitted>" "<sysroot>/lib/rustlib/aarch64-apple-darwin/lib/{libtest-*,libgetopts-*,librustc_std_workspace_std-*}.rlib" "/Users/kyle/gits/opte/target/debug/deps/{libopteadm-365b1ba283663929,libopte_ioctl-ec73f2082b07ba92,liblibnet-1c2c77ac3d2ddb1f,libnvpair-6dc86fe845b11132,libnvpair_sys-9bd7932e5eb9f6e9,libforeign_types-3dce6089390e9620,libforeign_types_shared-77fb9b9db2cee26d,libcstr_argument-a53831ec6e13f798,libmemchr-33619545b020ef55,libdlpi-9f98e74998236a22,liblibdlpi_sys-0c8aab08c61b8bdc,librand-dbbb335a35a3fab5,libgetrandom-bd213879c05479aa,libchacha20-5649b0530d981c08,librand_core-c2ccdfcb98ff3e68,libcfg_if-6b8837d359ee828b,libwinnow-75d267d3def663a0,libsocket2-c977aadc61e7cdbb,librusty_doors-77a1a51f611d80c9,libtracing-7750d39837035b20,libpin_project_lite-3db1bee4b603df0d,libtracing_core-ca73bca8a0361545,libonce_cell-82e50bbb82b898c8,liboxnet-c359e5e1ae02009d,libnum_enum-d2b02f0c87d2cb81,libcolored-14fc39fecb288324,liboxide_vpc-ce90b18cc876a083,libuuid-895778b822d7224f,libopte-8fbafeb436cabf0b,libtabwriter-4e2e13b066b22ade,libunicode_width-5a93898b1f030899,libopte_api-edb2afa854871069,libipnetwork-c041f60fb38a281d,libpostcard-e54da477eeb68185,libcobs-31050e357a1f6c92,libthiserror-d8a2dfe6a9f0a7f4,libsmoltcp-4feb72d0fb960e40,libbitflags-40479940b6962cab,libheapless-e7bf3788707cc4b2,libhash32-b7036d40526e0211,libbyteorder-f3001c905a03ce09,libmanaged-3c5477148deded2b,libillumos_sys_hdrs-d9fd9ef250253295,libingot-b5936e350108346b,libserde-5bc39fee4fccf79c,libingot_types-90da199d45026094,libmacaddr-1bbf20022f12c91f,libzerocopy-f8047dd1e27ac2a5,libclap-b31326f050f8224c,libclap_builder-0eec8e91071de01f,libterminal_size-32742994f6403d18,librustix-882bf71fde09ecc2,libbitflags-bd56ca75dd0cc5f3,libserde_core-43790efcdeaab862,liberrno-d7eb5950cb5f3d1b,liblibc-a37d9fb37ed3dfa3,libstrsim-715d6bf37b650a64,libanstream-98c082f1caedd3b7,libanstyle_query-637a921aa6a1b79c,libis_terminal_polyfill-0c3e705e41d44585,libcolorchoice-1bb300b26935b0ed,libanstyle_parse-03cd85ddaf3adf20,libutf8parse-036f4648c4aa2008,libclap_lex-51244e83ec956518,libanstyle-13a3dfac71985607,libanyhow-94b2a8e67ceee692}.rlib" "<sysroot>/lib/rustlib/aarch64-apple-darwin/lib/{libstd-*,libpanic_unwind-*,libobject-*,libmemchr-*,libaddr2line-*,libgimli-*,libcfg_if-*,librustc_demangle-*,libstd_detect-*,libhashbrown-*,librustc_std_workspace_alloc-*,libminiz_oxide-*,libadler2-*,libunwind-*,liblibc-*,librustc_std_workspace_core-*,liballoc-*,libcore-*,libcompiler_builtins-*}.rlib" "-lkstat" "-lnvpair" "-lzfs" "-ldlpi" "-liconv" "-lSystem" "-lc" "-lm" "-arch" "arm64" "-mmacosx-version-min=11.0.0" "-L" "/usr/local/zfs/lib" "-o" "/Users/kyle/gits/opte/target/debug/deps/opteadm-5b67c1f821efffe9" "-Wl,-dead_strip" "-nodefaultlibs"
  = note: some arguments are omitted. use `--verbose` to show all linker arguments
  = note: ld: warning: search path '/usr/local/zfs/lib' not found
          ld: library 'kstat' not found
          clang: error: linker command failed with exit code 1 (use -v to see invocation)


error: could not compile `opteadm` (bin "opteadm" test) due to 1 previous error
kyle@Mac opte % cargo t -p opte-ioctl
   Compiling opte v0.1.0 (/Users/kyle/gits/opte/lib/opte)
   Compiling ingot v0.1.1
   Compiling opte-api v0.1.0 (/Users/kyle/gits/opte/crates/opte-api)
   Compiling oxide-vpc v0.1.0 (/Users/kyle/gits/opte/lib/oxide-vpc)
   Compiling opte-ioctl v0.1.0 (/Users/kyle/gits/opte/lib/opte-ioctl)
error: linking with `cc` failed: exit status: 1
  |
  = note:  "cc" "/Users/kyle/gits/opte/target/debug/deps/rustc3LJzqY/symbols.o" "<4 object files omitted>" "<sysroot>/lib/rustlib/aarch64-apple-darwin/lib/{libtest-*,libgetopts-*,librustc_std_workspace_std-*}.rlib" "/Users/kyle/gits/opte/target/debug/deps/{liblibnet-1c2c77ac3d2ddb1f,libnvpair-6dc86fe845b11132,libnvpair_sys-9bd7932e5eb9f6e9,libforeign_types-3dce6089390e9620,libforeign_types_shared-77fb9b9db2cee26d,libcstr_argument-a53831ec6e13f798,libmemchr-33619545b020ef55,libdlpi-9f98e74998236a22,liblibdlpi_sys-0c8aab08c61b8bdc,librand-dbbb335a35a3fab5,libgetrandom-bd213879c05479aa,libchacha20-5649b0530d981c08,librand_core-c2ccdfcb98ff3e68,libcfg_if-6b8837d359ee828b,libwinnow-75d267d3def663a0,libsocket2-c977aadc61e7cdbb,librusty_doors-77a1a51f611d80c9,liblibc-a37d9fb37ed3dfa3,libtracing-7750d39837035b20,libpin_project_lite-3db1bee4b603df0d,libtracing_core-ca73bca8a0361545,libonce_cell-82e50bbb82b898c8,liboxnet-c359e5e1ae02009d,libnum_enum-d2b02f0c87d2cb81,libcolored-14fc39fecb288324,liboxide_vpc-31c3ebfe7483f6a1,libuuid-895778b822d7224f,libopte-df56da2da0dbc3bf,libopte_api-8f635a500bcb24ac,libpostcard-e54da477eeb68185,libcobs-31050e357a1f6c92,libthiserror-d8a2dfe6a9f0a7f4,libsmoltcp-4feb72d0fb960e40,libbitflags-40479940b6962cab,libheapless-e7bf3788707cc4b2,libhash32-b7036d40526e0211,libbyteorder-f3001c905a03ce09,libmanaged-3c5477148deded2b,libillumos_sys_hdrs-856c3c42578e557d,libingot-cea9f51d539a38bf,libserde-5bc39fee4fccf79c,libbitflags-80d7f1945a040152,libserde_core-43790efcdeaab862,libingot_types-90da199d45026094,libmacaddr-1bbf20022f12c91f,libzerocopy-f8047dd1e27ac2a5}.rlib" "<sysroot>/lib/rustlib/aarch64-apple-darwin/lib/{libstd-*,libpanic_unwind-*,libobject-*,libmemchr-*,libaddr2line-*,libgimli-*,libcfg_if-*,librustc_demangle-*,libstd_detect-*,libhashbrown-*,librustc_std_workspace_alloc-*,libminiz_oxide-*,libadler2-*,libunwind-*,liblibc-*,librustc_std_workspace_core-*,liballoc-*,libcore-*,libcompiler_builtins-*}.rlib" "-lkstat" "-lnvpair" "-lzfs" "-ldlpi" "-liconv" "-lSystem" "-lc" "-lm" "-arch" "arm64" "-mmacosx-version-min=11.0.0" "-L" "/usr/local/zfs/lib" "-o" "/Users/kyle/gits/opte/target/debug/deps/opte_ioctl-294210b331bcbae6" "-Wl,-dead_strip" "-nodefaultlibs"
  = note: some arguments are omitted. use `--verbose` to show all linker arguments
  = note: ld: warning: search path '/usr/local/zfs/lib' not found
          ld: library 'kstat' not found
          clang: error: linker command failed with exit code 1 (use -v to see invocation)


error: could not compile `opte-ioctl` (lib test) due to 1 previous error
warning: build failed, waiting for other jobs to finish...

I suspect this has probably broken due to a cargo update on libnet (and we obviously don't have MacOS in CI). This is probably a matter of making sure that the dependency is excluded on non-illumos environments?