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.
If we want to force gcloud to pick up the built version, we should remove this line (and the line above) so that the tests will fail on gke if we can't built kubectl. Otherwise we have a weird situation where sometimes we are using the kubectl built from source and other times we are using the kubectl packaged with gcloud.