Update main.yml

run the CI when pull request activity happens as well.

.github/workflows/main.yml

@@ -4,6 +4,9 @@ name: CI
 
 # Controls when the workflow will run
 on:
+  # Triggers the workflow when activity on a pull request in the workflow's repository occurs
+  pull_request:
+
   # Triggers the workflow on push or pull request events but only for the main branch
   push:
     branches: [ master ]