[ghaci] run github actions nightly job weekly on sunday midnight

.github/workflows/nightly.yaml

@@ -17,7 +17,7 @@ on:
       - main
   pull_request:
   schedule:
-    - cron: '* 10 * * *'
+    - cron: '* 0 * * 0'
 
 # This allows a subsequently queued workflow run to interrupt previous runs
 concurrency: