GKE: Adjust the PATH to the right kubectl for gcloud

Force kubectl to the git repo version if we detect a built version,
otherwise a system version. Previously, your actual PATH influenced
what kubectl was getting picked up, which violates POLA for a dev
environment.

Along the way: Add the root install of kubectl.
Actually, better yet, since we're allowing `GCLOUD=foo/bar/gcloud`, why not `KUBECTL=foo/bar/kubectl`? Seems like that would allow you to test from a system install, but it forces you to be conscious about it.