Move to flake based unstable import

modules/system/settings/nix/default.nix

@@ -1,4 +1,4 @@
-{ ... }:
+{ config, pkgs, unstable, ... }:
 {
   imports = [ ./gc ];
 
@@ -9,4 +9,9 @@
     ];
     auto-optimise-store = true;
   };
+
+  _module.args.pkgsUnstable = import unstable {
+    inherit (pkgs.stdenv.hostPlatform) system;
+    inherit (config.nixpkgs) config;
+  };
 }