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.
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