In case `target_user_ssh_key` points to an empty string, nixos-copy.sh
just doesn't set `IdentityFile=` at all.
This allows using deploy-nixos without any explicitly passed ssh keys,
but picking up whatever ssh setup the user has configured locally.
Change-Id: If335ce8434627e61da13bf6923b9767085af08a5
Reviewed-on: https://cl.tvl.fyi/c/depot/+/8576
Autosubmit: flokli <flokli@flokli.de>
Reviewed-by: tazjin <tazjin@tvl.su>
Tested-by: BuildkiteCI
14a8ea9eab