glibcLocales: cleaner environment handling

- make pkgs attribute null on unsupported Darwin platform
- add setup hook to provide much-duplicated environment variable

There are still some locations where LOCALE_ARCHIVE is set manually, these are special cases not using the default builder.

This is needed to be able to install nixops on Darwin.

@edolstra @joelteon @copumpkin ? Note that this is a PR for the darwin-clang-stdenv branch.
These parentheses are not necessary.