Schedule deploy once per month (instead of per day)

This reduces the API token quota

Author: mre

.github/workflows/ci.yml

@@ -6,7 +6,7 @@ on:
   pull_request:
     branches: [master]
   schedule:
-    - cron: "0 18 * * *"
+    - cron: "0 18 1 * *"
   repository_dispatch:
   workflow_dispatch: