`nixos-rebuild/deploy.sh` script enable ssh authentication with a given private key
through the `SSH_KEY` environment variable.
Add additional variable for the private key used for the deployment.
To encourage the use of ssh-agent and discourage the storage of deployment keys in the terraform state
we do not set the install ssh key as the default for the deployment key.
Co-authored-by: Jörg Thalheim <Mic92@users.noreply.github.com>
