fix ZONE parameter

Author: tonglil

auth-gke.sh

@@ -2,10 +2,9 @@
 set -euo pipefail
 
 main() {
-  _trace gcloud container clusters get-credentials "$CLUSTER" --project "$PROJECT" --zone "${LOCATION:-$ZONE}"
+  _trace gcloud container clusters get-credentials "$CLUSTER" --project "$PROJECT" --zone "${LOCATION:-${ZONE:-}}"
 }
 
-
 # Display the underlying command
 _trace() {
   cmd=($@)