Add `fetch` command to try to populate the cache from the public store

I wanted to download the latest version of Git in the Public Store without having to built it locally if it wasn't available for my architecture. This command would do just that: download if possible, else just say we have to build it.

$ warp fetch https://rules.warp.build/toolchains/elixir
  Downloading https://rules.warp.build/toolchains/elixir
  Finished ...

$ warp fetch https://rules.warp.build/toolchains/git
  The target "https://rules.warp.build/toolchains/git" has never been built before.
  To build it run: warp build https://rules.warp.build/toolchains/git