nixos-rebuild: Allow local builds when --target-host is used again

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.

nixos/doc/manual/from_md/release-notes/rl-2305.section.xml

@@ -256,6 +256,15 @@
           been changed to <literal>null</literal>.
         </para>
       </listitem>
+      <listitem>
+        <para>
+          The <literal>--target-host</literal> and
+          <literal>--build-host</literal> options of
+          <literal>nixos-rebuild</literal> no longer treat the
+          <literal>localhost</literal> value specially – to build
+          on/deploy to local machine, omit the relevant flag.
+        </para>
+      </listitem>
       <listitem>
         <para>
           The <literal>nix.readOnlyStore</literal> option has been

nixos/doc/manual/man-nixos-rebuild.xml

@@ -532,15 +532,15 @@
     <listitem>
      <para>
       Specifies the NixOS target host. By setting this to something other than
-      <replaceable>localhost</replaceable>, the system activation will happen
+      an empty string, the system activation will happen
       on the remote host instead of the local machine. The remote host needs to
       be accessible over ssh, and for the commands <option>switch</option>,
       <option>boot</option> and <option>test</option> you need root access.
      </para>
 
      <para>
-      If <option>--build-host</option> is not explicitly specified, building
-      will take place locally.
+      If <option>--build-host</option> is not explicitly specified or empty,
+      building will take place locally.
      </para>
 
      <para>

nixos/doc/manual/release-notes/rl-2305.section.md

@@ -67,6 +67,8 @@ In addition to numerous new and upgraded packages, this release has the followin
 
 - In `mastodon` it is now necessary to specify location of file with `PostgreSQL` database password. In `services.mastodon.database.passwordFile` parameter default value `/var/lib/mastodon/secrets/db-password` has been changed to `null`.
 
+- The `--target-host` and `--build-host` options of `nixos-rebuild` no longer treat the `localhost` value specially – to build on/deploy to local machine, omit the relevant flag.
+
 - The `nix.readOnlyStore` option has been renamed to `boot.readOnlyNixStore` to clarify that it configures the NixOS boot process, not the Nix daemon.
 
 ## Other Notable Changes {#sec-release-23.05-notable-changes}

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

@@ -148,10 +148,6 @@ if [[ -n "$SUDO_USER" || -n $remoteSudo ]]; then
     maybeSudo=(sudo --preserve-env="$preservedSudoVars" --)
 fi
 
-if [[ -z "$buildHost" && -n "$targetHost" ]]; then
-    buildHost="$targetHost"
-fi
-
 # log the given argument to stderr if verbose mode is on
 logVerbose() {
     if [ -n "$verboseScript" ]; then