No support for ssh-ng:// protocol

[knedlsepp]

The most recent version of nix supports coloured build outputs even when doing remote builds by specifying the build machines with the ssh-ng:// protocol in the /etc/nix/machines file.
This in itself works great, however I also have configured a hydra instance on that particular machine. Hydra's parser of the machines file extracts 'ssh-ng://hostname' as the hostname, which results in connection failure. So there currently is no way to have both the coloured build logs on manually triggered builds and a hydra instance running on the same machine. (Not trying to achieve coloured hydra logs here, but that issue is kind of related: #148)

[Ericson2314]

NixOS/nix#10748 / #1444 takes a big step in this direction. Hydra is finally weened off ad-hoc things, and made to go through the LegacySSHStore interface. Yes, LegacySSHStore is giving some new interfaces just for Hydra, but this how we figure out what the needed interfaces are. We can them implement those same interfaces on SSHStore (ssh-ng://), and then switch over to using that instead.