[dune-configurator] PKG_CONFIG_PATH is ignored on macos

Expected Behavior

When providing custom PKG_CONFIG_PATH variable dependencies

Actual Behavior

PKG_CONFIG_PATH is ignored in favor of brew https://github.com/ocaml/dune/blob/main/otherlibs/configurator/src/v1.ml

Reproduction

1. Get a mac
2. Add a folder with custom pkg-config, e.g. vcpkg install libpng
3. Run PKG_CONFIG= dune build
4. Observe error

Specifications

• Version of dune (output of dune --version): 3.16.0
• Version of ocaml (output of ocamlc --version): 5.2.0
• Operating system (distribution and version): MacOS 14.5

Additional information

Despite the code looks correct it is for some reason still not running, would be also good to log the modified PKG_CONFIG_PATH