nixos-rebuild: allow to override builders

Since nix 2.0 the no-build-hook option was replaced by the builders options
that allows to override remote builders ad-hoc.
Since it is useful to disable remote builders updating nixos without network,
this commit reintroduces the option.

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

@@ -66,6 +66,10 @@
    <arg>
     <option>--rollback</option>
    </arg>
+   <arg>
+     <option>--builders</option>
+     <replaceable>builder-spec</replaceable>
+   </arg>
    <sbr />
    <arg>
     <group choice='req'>
@@ -315,6 +319,27 @@ $ ./result/bin/run-*-vm
      </para>
     </listitem>
    </varlistentry>
+   <varlistentry>
+     <term>
+       <option>--builders</option>
+       <replaceable>builder-spec</replaceable>
+     </term>
+     <listitem>
+       <para>
+         Allow to specify remote builders ad-hoc for building the new system.
+         This requires the user executing <command>nixos-rebuild</command> (usually
+         root) to be configured as a trusted user in the Nix daemon. This can be
+         achived by using the <literal>nix.trustedUsers</literal> NixOS option.
+         Examples values for that option are described in the
+         <literal>Remote builds chapter<literal> in the Nix manual,
+         (i.e. <command>--builders "ssh://bigbrother x86_64-linux"</command>).
+         By specifying an empty string existing builders specified in
+         <filename>/etc/nix/machines</filename> can be ignored:
+         <command>--builders ""</command> for example when they are not
+         reachable due to network connectivity.
+       </para>
+     </listitem>
+   </varlistentry>
    <varlistentry>
     <term>
      <option>--profile-name</option>

nixos/modules/installer/tools/nixos-rebuild.sh

@@ -53,11 +53,11 @@ while [ "$#" -gt 0 ]; do
         repair=1
         extraBuildFlags+=("$i")
         ;;
-      --max-jobs|-j|--cores|-I)
+      --max-jobs|-j|--cores|-I|--builders)
         j="$1"; shift 1
         extraBuildFlags+=("$i" "$j")
         ;;
-      --show-trace|--no-build-hook|--keep-failed|-K|--keep-going|-k|--verbose|-v|-vv|-vvv|-vvvv|-vvvvv|--fallback|--repair|--no-build-output|-Q|-j*)
+      --show-trace|--keep-failed|-K|--keep-going|-k|--verbose|-v|-vv|-vvv|-vvvv|-vvvvv|--fallback|--repair|--no-build-output|-Q|-j*)
         extraBuildFlags+=("$i")
         ;;
       --option)