Adjust the PATH to the right kubectl for gcloud, add KUBECTL_PATH override

Force kubectl to the git repo version if we detect a built version.
Add a KUBECTL_PATH override so that system/home directory versions of
kubectl can be used if desired. Remove existing $HOME inference.
Merge pull request #3092 from zmerlynn/gke-kubectl-path

GKE: Adjust the PATH to the right kubectl for gcloud