ci: restrict build workflow to PRs targeting main

Only run build workflow when pull requests target the main branch,
preventing unnecessary builds on feature branch pushes.

Author: caidanw

.github/workflows/build.yml

@@ -1,7 +1,9 @@
 name: Build
 
 on:
-  push:
+  pull_request:
+    branches:
+      - main
 
 jobs:
   test: