feat: migrate to nix flake

system/default.nix

@@ -5,7 +5,6 @@
     ./hardened.nix
     ./i18n.nix
     ./nix.nix
-    ./nur.nix
     ./vpn.nix
   ];
 

system/nix.nix

@@ -1,6 +1,12 @@
 { config, pkgs, ... }:
 
 {
+  # for flakes support
+  nix.package = pkgs.nixUnstable;
+  nix.extraOptions = ''
+    experimental-features = nix-command flakes
+  '';
+
   nix.autoOptimiseStore = true;
   nix.gc = {
     automatic = true;

system/nur.nix

@@ -1,9 +0,0 @@
-{ config, pkgs, ... }:
-
-{
-  nixpkgs.config.packageOverrides = pkgs: {
-    nur = import (builtins.fetchTarball "https://github.com/nix-community/NUR/archive/master.tar.gz") {
-      inherit pkgs;
-    };
-  };
-}