Motivation for this change

SDDM has a number of other important files in there, which won't be linked otherwise. Also, this way other packages can now contribute translations etc.

(This currently doesn't bite you if you use most DEs because of #47173)

I tested this by building a system without my DE and it now links the paths properly where it didn't before.

@jtojnar finally remembered to do this.

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 nox --run "nox-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)
• Fits CONTRIBUTING.md.