nixos/profiles/nix-builder-vm: allow the system derivation to be substituted (#407513)

    # TODO system.switch.enable = false;?

    system.disableInstallerTools = true;

    # Allow the system derivation to be substituted, so that

    # users are less likely to run into a state where they need

    # the builder running to build the builder if they just want

    # to make a tweak that only affects the macOS side of things,

    # like changing the QEMU args.

    #

    # TODO(winter): Move to qemu-vm? Trying it here for now as a

    # low impact change that'll probably improve people's experience.

    #

    # (I have no clue what is going on in https://github.com/nix-darwin/nix-darwin/issues/1081

    # though, as this fix would only apply to one person in that thread... hopefully someone

    # comes across with a reproducer if this doesn't do it.)

    system.systemBuilderArgs.allowSubstitutes = true;

    nix.settings = {

      min-free = cfg.min-free;