Trigger on pull_request for testing

One workaround is to trigger on pull_request - let's see if that works

Signed-off-by: Elizabeth Byerly <ebyerly@google.com>

Author: ebyerly

.github/workflows/check-envoy-commits.yml

@@ -1,6 +1,7 @@
 name: "Check Envoy commits for non-trivial merges"
 
 on:
+  pull_request:
   schedule:
     - cron: "0 */4 * * *"
   workflow_dispatch: {}