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.
I think that's a verify_prereqs thing to do, not an every-time-you-run kubectl.sh thing to do. :) I did consider it, though.