flake: Restore packaging-overriding check

The underlying bug seems to have been fixed in diffoscope 293 [1] [2].
Our nixpkgs input has 295.

[1]: https://github.com/NixOS/nixpkgs/pull/393381#issuecomment-2766703347
[2]: https://diffoscope.org/news/diffoscope-292-released/

flake.nix

@@ -233,9 +233,6 @@
             This shouldn't build anything significant; just check that things
             (including derivations) are _set up_ correctly.
           */
-          # Disabled due to a bug in `testEqualContents` (see
-          # https://github.com/NixOS/nix/issues/12690).
-          /*
           packaging-overriding =
             let
               pkgs = nixpkgsFor.${system}.native;
@@ -254,7 +251,6 @@
                   # Same for all components; nix-util is an arbitrary pick
                   (nix.appendPatches [ pkgs.emptyFile ]).libs.nix-util.src;
               };
-          */
         }
         // (lib.optionalAttrs (builtins.elem system linux64BitSystems)) {
           dockerImage = self.hydraJobs.dockerImage.${system};