budgie-desktop-with-plugins: Populate GI_TYPELIB_PATH from plugins

Things happen to work because budgie-desktop depends on libpeas,
which propagates gobject-introspection, and so far we only package
one Python budgie plugin (budgie-media-player-applet) and that
plugin does not require deps that are not available in budgie-desktop.

Let's actually handle GI_TYPELIB_PATH in -with-plugins wrapper so
the paths actually come from plugins's buildInputs.

pkgs/by-name/bu/budgie-desktop-with-plugins/package.nix

@@ -2,6 +2,7 @@
   lib,
   stdenv,
   glib,
+  gobject-introspection,
   xorg,
   wrapGAppsHook3,
   budgie-desktop,
@@ -20,6 +21,7 @@ stdenv.mkDerivation {
 
   nativeBuildInputs = [
     glib
+    gobject-introspection.setupHook
     wrapGAppsHook3
   ];