nixos-rebuild: default buildHost to localhost

Prior to #119540, the toplevel output was built
locally then pushed to the remote host when using
the command:

nixos-rebuild --flake <flake> --target-host remote.host

However, buildHost would default to targetHost when
only --target-host was supplied. This caused the .drv
closure to always be transferred unless --build-host=localhost
is supplied.

This change restores the previous build local default behavior.
TargetHost can still build the outputs, if explicitly mentioned:

nixos-rebuild --flake <flake> \
  --target-host remote.host \
  --build-host  remote.host

pkgs/os-specific/linux/nixos-rebuild/nixos-rebuild.sh

@@ -26,7 +26,7 @@ upgrade=
 upgrade_all=
 repair=
 profile=/nix/var/nix/profiles/system
-buildHost=
+buildHost=localhost
 targetHost=
 maybeSudo=()