CI: Run on PRs and merges to main

This is a best practice from the Vox Pupuli community. It ensures that
the pipeline runs for all PRs. Because PRs can be outdated before a
merge, the pipeline will run again after the merge.

Maybe you want to adopt this pattern.

.github/workflows/ci.yml

@@ -1,8 +1,10 @@
 name: CI
 
 on:
-  pull_request:
+  pull_request: {}
   push:
+    branches:
+      - main
 
 jobs:
   test: