Disable packaging-overriding

Fixes #12690.

flake.nix

@@ -230,6 +230,9 @@
             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.${system}.native;
@@ -248,6 +251,7 @@
                     # Same for all components; nix-util is an arbitrary pick
                     (nix.appendPatches [ pkgs.emptyFile ]).libs.nix-util.src;
                 };
+          */
         }
         // (lib.optionalAttrs (builtins.elem system linux64BitSystems)) {
           dockerImage = self.hydraJobs.dockerImage.${system};