GH action: merge staging(-next): fix cron

It went every minute past every 6th hour, which was too often.
Now it should go on the zeroth minute every 6th hour.

.github/workflows/merge-staging.yml

@@ -4,7 +4,7 @@ on:
   schedule:
     # * is a special character in YAML so you have to quote this string
     # Merge every 6 hours
-    - cron:  '* */6 * * *'
+    - cron:  '0 */6 * * *'
 
 jobs:
   sync-branch: