macos-builder: readd Nix CLI for debugging (#347205)

    # The linux builder is a lightweight VM for remote building; not evaluation.

    nix.channel.enable = false;

    # remote builder uses `nix-daemon` (ssh-ng:) or `nix-store --serve` (ssh:)

    # --force: do not complain when missing

    # TODO: install a store-only nix

    #       https://github.com/NixOS/rfcs/blob/master/rfcs/0134-nix-store-layer.md#detailed-design

    environment.extraSetup = ''

      rm --force $out/bin/{nix-instantiate,nix-build,nix-shell,nix-prefetch*,nix}

    '';

    # Deployment is by image.

    # TODO system.switch.enable = false;?

    system.disableInstallerTools = true;