Run pull request workflow on merge to main

In order for the CI cache to work during the PR pipeline, the pipeline
also needs to run during the merge to main.

.github/workflows/pull-request.yml

@@ -1,6 +1,11 @@
 name: Pull Request
 
-on: [pull_request]
+on:
+    pull_request:
+    # run CI/CD against main as well, to generate cache
+    push:
+        branches:
+            - main
 
 jobs:
     test: