This is a followup of #148921, to allow local builds when
`--target-host` is used again. It also documents the change in
behavior, regarding the specialty of the `localhost` value.
By removing the special handling of an empty `buildHost` and non empty
`targetHost`, this change also slightly alters the behavior of
`nixos-rebuild`.
Originally by specifying `--target-host target --build-host ""`, the
now removed special case would transform those arguments to
`--target-host target --build-host target`.
Now the empty `--build-host` would result in a local build.
cc4de1aa3a