Motivation for this change

Fix NixOS manual build failure introduced by #83104.

manual-combined.xml:82287: element link: validity error : IDREF attribute linkend references an unknown ID "opt-nix.buildMachine"
 82283       </para>
 82284     </listitem>
 82285     <listitem>
 82286      <para>
 82287       The <link linkend="opt-nix.buildMachine">nix.buildMachine</link> option is now type-checked.
 82288       There are no functional changes, however this may require updating some configurations to use correct types for all attributes.
 82289      </para>

manual-combined.xml fails to validate

Things done

• Tested with nix build -f ./nixos --arg configuration '{ config = { fileSystems."/".fsType = "tmpfs"; boot.loader.systemd-boot.enable = true; };}' system`
• Tested using sandboxing (nix.useSandbox on NixOS, or option sandbox in nix.conf on non-NixOS linux)
• Built on platform(s)
  • NixOS
  • macOS
  • other Linux distributions
• Tested via one or more NixOS test(s) if existing and applicable for the change (look inside nixos/tests)
• Tested compilation of all pkgs that depend on this change using nix-shell -p nixpkgs-review --run "nixpkgs-review wip"
• Tested execution of all binary files (usually in ./result/bin/)
• Determined the impact on package closure size (by running nix path-info -S before and after)
• Ensured that relevant documentation is up to date
• Fits CONTRIBUTING.md.

CC @Valodim @Ericson2314