ci: Support PR trigger on CD workflow

.github/workflows/cd_release_please.yml

@@ -2,6 +2,8 @@ name: "CD: Release and Publish"
 
 on:
   workflow_dispatch:
+  pull_request:
+    types: [opened, synchronize, reopened]
   push:
     # Run on changes and let release-please create
     # PRs for version bumps and new releases
@@ -45,7 +47,7 @@ jobs:
         uses: lekman/release-please-dotnet@v1
         with:
           manifest: ".release-please-manifest.json"
-          branch: ${{ github.head_ref }}
+          branch: ${{ github.event.pull_request.head.ref || github.ref_name }}
 
   publish:
     name: Publish