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, pkgs.

Motivation for this change

Things done

• Tested using sandboxing (nix.useSandbox on NixOS, or option sandbox in nix.conf on non-NixOS)
• Built on platform(s)
  • NixOS
  • macOS
  • other Linux distributions
• Tested via one or more NixOS test(s) if existing and applicable for the change (look inside nixos/tests)
• Tested compilation of all pkgs that depend on this change using nix-shell -p nix-review --run "nix-review wip"
• Tested execution of all binary files (usually in ./result/bin/)
• Determined the impact on package closure size (by running nix path-info -S before and after)
• Assured whether relevant documentation is up to date
• Fits CONTRIBUTING.md.