gnome.updateScript: expose canonical attrPath

Without this, `update.nix` will discover `libxml2` as `emscriptenPackages.libxml2`.
But since that instantiates a different, and much less common derivation,
it will be erroneously considered a leaf package in topological order of packages.

Changed files
+1
pkgs
desktops
gnome
+1
pkgs/desktops/gnome/update.nix
···
supportedFeatures = [
"commit"
];
+
inherit attrPath;
}