More config-go.sh cleanup

Now the environment variables.

Document the ones that are supposed to be public, make sure they're exported.

Run setup of output/go/ tree in a subshell to avoid leaking the variables.

Simplify finding absolute path to the top of the tree (`${KUBE_REPO_ROOT}`) by using cd & pwd in a subshell instead of using readlink on Linux and a hack on Mac.