Update Gemini PR review workflow to use ubuntu-latest and add GCP authentication

Author: yongruilin

.github/workflows/gemini-pr-review.yml

@@ -6,7 +6,7 @@ on:
 
 jobs:
   gemini_review:
-    runs-on: arc-runner-set
+    runs-on: ubuntu-latest
 
     steps:
       - name: Checkout code
@@ -22,6 +22,11 @@ jobs:
       - name: Install dependencies
         run: pip install google-generativeai requests PyGithub google-cloud-storage
 
+      - name: Authenticate to Google Cloud
+        uses: google-github-actions/auth@v1
+        with:
+          credentials_json: ${{ secrets.GCP_SERVICE_ACCOUNT_KEY }}
+
       - name: Run Gemini review script
         env:
           GEMINI_API_KEY: ${{ secrets.GEMINI_API_KEY }}