Run PR workflow on merge to main

Since we don't require branches to be up to date before merge, it would
be a good idea to run CI on `main` after a merge. (Issue noted by
@cpcloud)

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 }}