nixpkgs: show definition files in config assertion

Backport of Nixpkgs commit 6d9dfef94ffd59a327573eea7bc709a84c44b3d2.

Co-authored-by: Matt Sturgeon <matt@sturgeon.me.uk>

modules/nix/nixpkgs.nix

@@ -306,6 +306,9 @@ see https://github.com/NixOS/nixpkgs/issues/278001
 
           Current value:
           ${lib.generators.toPretty { multiline = true; } cfg.config}
+
+          Defined in:
+          ${lib.concatMapStringsSep "\n" (file: "  - ${file}") opt.config.files}
         '';
       }
     ];