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.
Yeah, I actually debated that. We can either force the git version, or allow the gcloud version. As written, this would actually let you do a "make clean" and use the gcloud version for e2e's, though, and there's no easy way to do that otherwise.