github: Fix ref in cache cleanup workflow

.github/workflows/cache-cleanup.yml

@@ -12,5 +12,5 @@ jobs:
       - env:
           GH_TOKEN: ${{ github.token }}
         # github.ref for deletes is the default branch, so we get the ref from
-        # the event
-        run: gh cache delete --repo ${{ github.repository }} -r ${{ github.event.ref }} --all --succeed-on-no-caches
+        # the event, which is not the full ref
+        run: gh cache delete --repo ${{ github.repository }} -r refs/heads/${{ github.event.ref }} --all --succeed-on-no-caches