pkgs.nixos: Expose configuration directly

The pkgs.nixos used to only expose system.build, which kind of made
sense in theory, but asking everyone to write modules when to want
to pull something out of a NixOS configuration is just not realistic.
In fact it's very inconvenient when you're trying to debug something.

This adds the config, options and anything that eval-config.nix
produces. Although this introduces the potential for attributes
of eval-config.nix output to shadow system.build, I don't expect
naming collisions to be commonplace, or to remain undetected during
evaluation. Also such an error should be easy to resolve, by explicitly
querying (pkgs.nixos {}).config.system.build.X, or by renaming X
to something other than config, options, extraArgs.