[CI] Run PR workflow on merge to main (#572)

Author: gmarkall

.github/workflows/pr.yaml

@@ -7,6 +7,7 @@ on:
   push:
     branches:
       - "pull-request/[0-9]+"
+      - "main"
 
 concurrency:
   group: ${{ github.workflow }}-${{ github.ref }}-${{ github.event_name }}