gnome.updateScript: Return explicitly set attr path

The update script currently requires attribute to be passed.
Let’s return it using the commit feature.
Otherwise, `update.nix` would use the attribute it first found
for the commit message, which can be something like `emscriptenPackages.libxml2`.

pkgs/desktops/gnome/update.nix

···
 
     report = [
         {
+            "attrPath": attr_path,
             "commitBody": f"https://gitlab.gnome.org/GNOME/{package_name}/-/compare/{package_version}...{latest_tag}",
         },
     ]