lib/nixos/eval-config.nix: Fix extraArgs

[infinisil]

Motivation for this change

Fixes a mistake in #148315 that
caused #148343 (comment)

Tested with this command:

nix-instantiate nixos/lib/eval-config.nix --arg modules '[ ({ x, ... }: { environment.etc.x.text = x; fileSystems."/".device = "d"; boot.loader.grub.device = "nodev"; }) ]' --arg extraArgs '{ x = "x"; }' -A config.system.build.toplevel

Which previously threw an error:

error: attribute 'x' missing, at /home/infinisil/src/nixpkgs/lib/modules.nix:385:28
(use '--show-trace' to show detailed location information)

But with this commit works, and gives the intended warning for extraArgs:

trace: warning: The extraArgs argument to eval-config.nix is deprecated. Please set config._module.args instead.
warning: you did not specify '--add-root'; the result might be removed by the garbage collector
/nix/store/hg5019xr1alcn76dbm38c17ci6x6pz3a-nixos-system-nixos-22.05pre-git.drv

Ping @roberth

Things done

• Built on platform(s)
  • x86_64-linux
  • aarch64-linux
  • x86_64-darwin
  • aarch64-darwin
• For non-Linux: Is sandbox = true set in nix.conf? (See Nix manual)
• Tested, as applicable:
  • NixOS test(s) (look inside nixos/tests)
  • and/or package tests
  • or, for functions and "core" functionality, tests in lib/tests or pkgs/test
  • made sure NixOS tests are linked to the relevant packages
• Tested compilation of all packages that depend on this change using nix-shell -p nixpkgs-review --run "nixpkgs-review rev HEAD". Note: all changes have to be committed, also see nixpkgs-review usage
• Tested basic functionality of all binary files (usually in ./result/bin/)
• 22.05 Release Notes (or backporting 21.11 Release notes)
  • (Package updates) Added a release notes entry if the change is major or breaking
  • (Module updates) Added a release notes entry if the change is significant
  • (Module addition) Added a release notes entry if adding a new NixOS module
  • (Release notes changes) Ran nixos/doc/manual/md-to-db.sh to update generated release notes
• Fits CONTRIBUTING.md.

[roberth]

I remember checking this, but I must have checked the wrong file. :(
Thanks for fixing. Let's wait for ofborg before merging, to make sure it doesn't fail because of new warnings, warnings that aren't allowed.