Trigger CI build on pull requests.

Also, use default 'run-name' for more informative message.

(cherry picked from commit da2e4eb11b)

.github/workflows/ubuntu.yml

@@ -1,8 +1,10 @@
 name: Ubuntu build
-run-name: ${{ github.actor }} is triggering pipeline
+
 on:
   push:
-  workflow_dispatch:
+    branches:
+    - main
+  pull_request:
 
 jobs:
   build-module: