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 probably overkill, but it would be a bit more resilient to actually run kubectl (similarly to how you run gcloud preview in pr #3082) rather than just check the executable bit.