[ci] Run automatic checks weekly monday morning

.github/workflows/main.yml

   pull_request:
   # for now, CRON jobs only run on the default branch of the repo (i.e. usually on master)
   schedule:
-    # * is a special character in YAML so you have to quote this string
-    - cron: "0 4 * * *"
+    # weekly Monday morning
+    - cron: "0 6 * * 1"
 
 name: CI by {tic}