When using running `tailscale up` in the AuthKey flow process, the tailscale client immediately enters PollMap after registration - avoiding a race condition.
When using the web auth (up -> go to the Control website -> CLI `register`) the client is polling checking if it has been authorized. If we immediately ask for the client IP, as done in CreateHeadscaleEnv() we might have the client in NotReady status.
This method provides a way to wait for the client to be ready.
Signed-off-by: Juan Font Alonso <juanfontalonso@gmail.com>
