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.
This is a good solution, except that `PATH` will now grow longer and longer for every kubectl call. Is this worth worry about? I'd looked at variable length limits for shells the other day and it seemed pretty generous, so this probably won't break anyone, but it is kind of gross.