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.
Another nit: do we need quotes here?

I think none of us have spaces in our paths, so in guarding against them I never know if I'm being unnecessarily cautious.