Add missing variable

Author: jakweg

ci/cloudbuild/release-and-test.yaml

@@ -67,6 +67,7 @@ steps:
     args: ["./hack/prow/e2e-test.sh"]
     env:
       - "GCP_PROJECT_ID=$PROJECT_ID"
+      - "REF_NAME=${REF_NAME}"
       - "BUILD_ID=$BUILD_ID"
       - "RELEASE_BUCKET=$_RELEASE_BUCKET"
       - "EXPORT_BUCKET=$_EXPORT_BUCKET"