schema: allow the gnome extension (#3920)

core22 is currently using the legacy implementation of remote-build
and needs to allow gnome in order to work.

The legacy code is safe, it will gracefully fail if used with the gnome
extension at loading time

LP: #1986962

Signed-off-by: Sergio Schvezov <sergio.schvezov@canonical.com>

schema/snapcraft.json

@@ -909,6 +909,7 @@
                                     "flutter-beta",
                                     "flutter-dev",
                                     "flutter-master",
+                                    "gnome",
                                     "gnome-3-28",
                                     "gnome-3-34",
                                     "gnome-3-38",