gnome.updateScript: allow updating to unstable releases

It is now possible to update to GNOME pre-release by simply running:

GNOME_UPDATE_STABILITY=unstable nix-shell maintainers/scripts/update.nix --arg predicate '(path: pkg: pkg.updateScript.name or null == "gnome-update-script")' --argstr keep-going true --argstr commit true

pkgs/desktops/gnome/update.nix

@@ -21,7 +21,7 @@ let
     package_version="$3"
     version_policy="$4"
 
-    flvFlags=("$package_name" "$version_policy" "stable")
+    flvFlags=("$package_name" "$version_policy" "''${GNOME_UPDATE_STABILITY:-stable}")
 
     if (( $# >= 5 )); then
       upper_bound="$5"