glibcLocales: cleaner environment handling

A continuation/re-imagining of #4318, trying to make it cleaner.

- make pkgs attribute null on unsupported Darwin platform
- add setup hook to provide much-duplicated environment variable

This way, builds should find the locale archive when they provide buildInputs.

My only problem is that I wasn't able to test this on Linux yet. On Darwin it works fine, since it just removes everything :grin:. I mocked the glibcLocales build and it did create the proper setup-hook with the correct path.