prepare-instance should persist and pass SSH connection

[fkorotkov]

I was investigating slower than usual cold startup times for ghcr.io/cirruslabs/macos-runner:sequoia VMs and noticed couple interesting things.

It seems we do try to establish SSH connection to the VM twice and both times it's taking around 5 seconds:

Fixing the missing wait-for-ssh in #803.

For a standby instance both SSH connection and agent uploads just takes hundreds of milliseconds.

It seems we might want to optimize these things by passing by the SSH connection from prepare instance or alternetevely remove this logic from there and add extra logic to standby preparation.