Reapply "Run on pull requests only again"

This reverts commit fb15f28.

.github/workflows/trigger_on_pull_request.yml

@@ -1,13 +1,6 @@
 name: PR Checks
 
-#on: [ pull_request ]
-on:
-  push:
-    branches:
-      - '*'         # matches every branch that doesn't contain a '/'
-      - '*/*'       # matches every branch containing a single '/'
-      - '**'        # matches every branch
-      - '!master'   # excludes master
+on: [ pull_request ]
 
 concurrency:
   group: ci-${{github.actor}}-${{github.head_ref || github.run_number}}-${{github.ref}}-trigger_on_pull_request