nixos/steam: fix evaluation failure when `fonts.packages` contains path

    fontPackages = lib.mkOption {

      type = lib.types.listOf lib.types.package;

      default = config.fonts.packages;

      defaultText = lib.literalExpression "fonts.packages";

      # `fonts.packages` is a list of paths now, filter out which are not packages

      default = builtins.filter lib.types.package.check config.fonts.packages;

      defaultText = lib.literalExpression "builtins.filter lib.types.package.check config.fonts.packages";

      example = lib.literalExpression "with pkgs; [ source-han-sans ]";

      description = ''

        Font packages to use in Steam.