gnome-shell: Fix default WM settings

gnome-shell contains GSettings schema overrides for Mutter.
Since we do not have global FHS path for schemas, we compile schemas
locally per package and overrides for another package have no effect.
As a workaround, let’s copy the the Mutter schema here so it ends up
being included in gnome-shell’s `gschemas.compiled`.

This also has an effect on `gnome.nixos-gsettings-overrides`,
which pulls in schemas and overrides from `gnome-shell`.

pkgs/by-name/gn/gnome-shell/package.nix

···
     rm data/theme/gnome-shell-{light,dark}.css
   '';
 
+  preInstall = ''
+    # gnome-shell contains GSettings schema overrides for Mutter.
+    schemadir="$out/share/glib-2.0/schemas"
+    mkdir -p "$schemadir"
+    cp "${glib.getSchemaPath mutter}/org.gnome.mutter.gschema.xml" "$schemadir"
+  '';
+
   postInstall = ''
     # Pull in WebP and JXL support for gnome-backgrounds.
     # In postInstall to run before gappsWrapperArgsHook.