Make Nixpkgs jobs unique

That is, there are now distinct jobs like ‘coreutils.x86_64-linux’ and
‘coreutils.x86_64-darwin’, rather than a single job ‘coreutils’ with
multiple builds.  This means that testing a job is simpler:

  $ nix-build pkgs/top-level/release.nix -A coreutils.x86_64-linux

See https://github.com/NixOS/hydra/issues/60 for the motivation.