🐛 Template: only run CI for push to main

In the current event trigger configuration for `ci.yml`, each CI job would run twice
(once for the `push` trigger, once for the `pull_request` one). Here we restrict the
`push` trigger to only run for the `main` branch.

Author: mbercx

template/.github/workflows/.ci.yml

@@ -1,6 +1,10 @@
 name: ci
 
-on: [push, pull_request]
+on:                       
+  push:
+    branches:
+      - main
+  pull_request:
 
 jobs:
   pre-commit: