Run PR validation workflow biweekly

Run pr-validation-yml twice a month on the 1st and 15th day. This ensures we detect problems soon which do not arise from code changes but from changes in the environments, e.g. changes on the actions runner agents, removed dependencies on nuget.org and things like that.

Author: ckadluba

.github/workflows/pr-validation.yml

@@ -4,6 +4,10 @@ on:
   pull_request:
     branches: [ dev, main ]
 
+  # Run every biweekly to discover failures due to environment changes
+  schedule:
+    - cron: '0 0 1,15 * *'
+  
   # Allows you to run this workflow manually from the Actions tab
   workflow_dispatch: