ci: separate CI (PR + main) from build (main only after merge)

.gitea/workflows/build.yaml

@@ -4,6 +4,9 @@ on:
   push:
     branches:
       - main
+  pull_request:
+    branches:
+      - main
 
 jobs:
   test:
@@ -23,6 +26,7 @@ jobs:
     name: Build & Push Image
     runs-on: rust
     needs: test
+    if: github.event_name == 'push' && github.ref == 'refs/heads/main'
     steps:
       - name: Checkout
         uses: actions/checkout@v4