Add cron schedule in the CI to run periodically even without commits

Author: f18m

.github/workflows/main.yml

@@ -5,6 +5,10 @@ on:
     branches: [ master ]
   pull_request:
     branches: [ master ]
+  # add a cron job to run every month -- this project is not very active, at least ensure there's a valid CI build every month
+  # this is also useful to check if something breaks e.g. due to infrastructure changes (e.g. Ubuntu OS)
+  schedule:
+    - cron: '0 0 1 * *'
 
 jobs:
   build_test_with_prometheus_support: