modules/nixos: use latest nixos-option to support flakes

modules/machine.nix

@@ -1,4 +1,4 @@
-{ lib, pkgs, ... }:
+{ lib, ... }:
 
 {
   imports = [ ./common.nix ];

modules/nixos/configs/nix.nix

@@ -1,4 +1,4 @@
-{ lib, inputs, config, ... }:
+{ lib, inputs, config, pkgs, ... }:
 
 let
   inherit (builtins) elem;
@@ -24,6 +24,9 @@ in
   };
 
   config = {
+    environment.systemPackages =
+      lib.optional config.system.tools.nixos-option.enable pkgs.unstable.nixos-option;
+
     nixpkgs.config.allowUnfreePredicate = lib.mkIf
       (cfg.allowUnfreePackages != [ ])
       (pkg: elem (lib.getName pkg) cfg.allowUnfreePackages);