flatpak: clear GDK_PIXBUF_MODULE_FILE

GDK_PIXBUF_MODULE_FILE is often set to a nix store path not available in a app
sandbox. This can cause some apps to fail launching, simply reset this env var
when running applications.

fixes https://github.com/NixOS/nixpkgs/issues/53441

pkgs/development/libraries/flatpak/default.nix

@@ -32,6 +32,7 @@ stdenv.mkDerivation rec {
     # patch taken from gtk_doc
     ./respect-xml-catalog-files-var.patch
     ./use-flatpak-from-path.patch
+    ./unset-env-vars.patch
   ];
 
   nativeBuildInputs = [