Set GOOGLE_APPLICATION_CREDENTIALS after activating

hack/jenkins/e2e-runner.sh

@@ -258,6 +258,8 @@ fi
 if [[ -f "${KUBEKINS_SERVICE_ACCOUNT_FILE:-}" ]]; then
   echo 'Activating service account...'  # No harm in doing this multiple times.
   gcloud auth activate-service-account --key-file="${KUBEKINS_SERVICE_ACCOUNT_FILE}"
+  # https://developers.google.com/identity/protocols/application-default-credentials
+  export GOOGLE_APPLICATION_CREDENTIALS="${KUBEKINS_SERVICE_ACCOUNT_FILE}"
   unset KUBEKINS_SERVICE_ACCOUNT_FILE
 elif [[ -n "${KUBEKINS_SERVICE_ACCOUNT_FILE:-}" ]]; then
   echo "ERROR: cannot access service account file at: ${KUBEKINS_SERVICE_ACCOUNT_FILE}"