Add write permisison to VertexAI Response Checking task (#6869)

The task began failing recently likely due to changes in global
permissions, this updates the workflow to explicitly have write
permissions to make comments.

Author: emilypgoogle

.github/workflows/check-vertexai-responses.yml

@@ -5,6 +5,8 @@ on: pull_request
 jobs:
   check-version:
     runs-on: ubuntu-latest
+    permissions:
+      pull-requests: write
     steps:
       - uses: actions/checkout@11bd71901bbe5b1630ceea73d27597364c9af683 # v4.2.2
       - name: Clone mock responses