fix unbound var error

hack/jenkins/minikube_cross_build_and_upload.sh

@@ -83,7 +83,7 @@ rm -rf out/buildroot
 # -R: recursive. strangely, this is not the default for sync.
 gsutil -m rsync -dJR out "gs://${bucket}/${ghprbPullId}"
 
-if [[ "${ghprbPullId}" -eq "master" ]]; then
+if [[ "${ghprbPullId}" -eq master ]]; then
   # Copy artifacts to known mirror location
   gsutil cp -R "gs://${bucket}/${ghprbPullId}/minikube-*" "gs://${bucket_mirror}"
 fi