Use pull_request triggers primarily for workflows

Instead of relying on push triggers in general, we now use pull_request
and reserve push triggers for main and release branches.

This makes the behaviour more consistent for users contributing from
repository forks. However, we no longer have a quick-feedback loop
for development.

.github/workflows/ci-check-no-dist-update.yml

@@ -3,10 +3,7 @@ name: CI-check-no-dist-update
 # Prohibit any change to 'dist/**' on a non-release branch
 on:
   workflow_dispatch:
-  push:
-    branches-ignore:
-      - 'main'
-      - 'release/**'
+  pull_request:
     paths:
       - 'dist/**'