Activate a service account if this is defined

hack/jenkins/e2e-runner.sh

@@ -151,6 +151,11 @@ if running_in_docker; then
     fi
 fi
 
+if [[ -n "${KUBEKINS_SERVICE_ACCOUNT_FILE:-}" ]]; then
+  echo 'Activating service account...'  # No harm in doing this multiple times.
+  gcloud auth activate-service-account --key-file="${KUBEKINS_SERVICE_ACCOUNT_FILE}"
+fi
+
 # Install gcloud from a custom path if provided. Used to test GKE with gcloud
 # at HEAD, release candidate.
 # TODO: figure out how to avoid installing the cloud sdk twice if run inside Docker.