revert to the once-a-month schedule

GitHub doesn't seem to like "once a day" either, so I guess I can only
test scheduled workflows once a month.

Author: gelisam

.github/workflows/ci.yml

@@ -11,7 +11,7 @@ on:
 
   # Build once a month, to detect missing upper bounds.
   schedule:
-    - cron: '10 2 * * *'
+    - cron: '0 0 1 * *'
 
 jobs:
   stack: