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.
The nut of it here is the `kubectl.sh` is called as if it were a command all the time, and not sourced. So it's always a separate process, meaning it's a single instance of a shell. The `export PATH=` affects everything downstream, but shouldn't virally infect the stuff upstream, e.g.:

```
zml@zml:~/kubernetes$ echo $PATH
/usr/local/google/home/zml/google-cloud-sdk/bin:/usr/local/google/home/zml/google-cloud-sdk/bin:/usr/local/google/home/zml/bin:/usr/lib/google-golang/bin:/usr/local/buildtools/java/jdk/bin:/usr/local/sbin:/usr/local/bin:/usr/sbin:/usr/bin:/sbin:/bin
zml@zml:~/kubernetes$ bash
zml@zml:~/kubernetes$ export PATH=foo/bar/blatz:$PATH
zml@zml:~/kubernetes$ echo $PATH
foo/bar/blatz:/usr/local/google/home/zml/google-cloud-sdk/bin:/usr/local/google/home/zml/google-cloud-sdk/bin:/usr/local/google/home/zml/google-cloud-sdk/bin:/usr/local/google/home/zml/bin:/usr/lib/google-golang/bin:/usr/local/buildtools/java/jdk/bin:/usr/local/sbin:/usr/local/bin:/usr/sbin:/usr/bin:/sbin:/bin
zml@zml:~/kubernetes$ exit
zml@zml:~/kubernetes$ echo $PATH
/usr/local/google/home/zml/google-cloud-sdk/bin:/usr/local/google/home/zml/google-cloud-sdk/bin:/usr/local/google/home/zml/bin:/usr/lib/google-golang/bin:/usr/local/buildtools/java/jdk/bin:/usr/local/sbin:/usr/local/bin:/usr/sbin:/usr/bin:/sbin:/bin