Now the user can pass `--build-host local`, to select the local machine as a build host, in which case no ssh is used.
This means the admin machine does not necessarily have ssh set up to itself, which was confusing for many users.
Also this makes it easier to re-use a well configured nix remote build setup which is only available on the local machine. Eg if `--build-host local` nix' defaults for remote builds on that machine will be utilized.
