Run esmvalbot test PR twice a month (#3976)

Author: bouweandela

.github/workflows/cron_esmvalbot_test.yml

@@ -4,9 +4,9 @@ on:
   # push:
   #   branches:
   #     - cron_esmvalbot_test
-  # scheduled once every 2 weeks
+  # scheduled twice a month (on the 1st and 16th)
   schedule:
-    - cron: '0 4 */14 * *'
+    - cron: '0 4 1,16 * *'
 
 # Required shell entrypoint to have properly configured bash shell
 defaults: