nixos/facter: fix conflict with `readOnlyPkgs` (#456076)

  ...

}:

{

  # Skip setting hostPlatform in test VMs where it's read-only

  # Tests have virtualisation.test options and import read-only.nix

  config.nixpkgs = lib.optionalAttrs (!(options ? virtualisation.test)) {

    hostPlatform = lib.mkIf (

      config.hardware.facter.report.system or null != null && !options.nixpkgs.pkgs.isDefined

    ) (lib.mkDefault config.hardware.facter.report.system);

  # Skip setting hostPlatform if it's read-only

  nixpkgs =

    lib.optionalAttrs

      (config.hardware.facter.report.system or null != null && !options.nixpkgs.hostPlatform.readOnly)

      {

        hostPlatform = lib.mkDefault config.hardware.facter.report.system;

      };

}