coverage: Run on pull request in a few cases

If we're changing the integration test wrapper or coverage.yml, let's
run the coverage workflow on PRs as well to make sure it doesn't break.

.github/workflows/coverage.yml

@@ -7,6 +7,14 @@ on:
     # Calculate coverage daily at midnight
     - cron:  '0 0 * * *'
 
+  pull_request:
+    branches:
+      - main
+      - v[0-9]+-stable
+    paths:
+      - .github/workflows/coverage.yml
+      - test/integration-test-wrapper.py
+
 permissions:
   contents: read