[ci] Run stale cache cleanup every 6 hours

Run this a bit more frequently so we don't thrash `main` caches as often.

.github/workflows/shared_cleanup_stale_branch_caches.yml

@@ -3,7 +3,8 @@
 name: (Shared) Cleanup Stale Branch Caches
 on:
   schedule:
-    - cron: 0 0 * * *
+    # Every 6 hours
+    - cron: 0 */6 * * *
   workflow_dispatch:
 
 permissions: {}