Limit branches for pull request target workflow (#42635)

(cherry picked from commit e3fdb7e)

.github/workflows/build-images.yml

@@ -21,6 +21,8 @@ run-name: >
   Build images for ${{ github.event.pull_request.title }} ${{ github.event.pull_request._links.html.href }}
 on:  # yamllint disable-line rule:truthy
   pull_request_target:
+    branches:
+      - main
 permissions:
   # all other permissions are set to none
   contents: read