More option simplification

system/nix/default.nix

@@ -0,0 +1,22 @@
+{ config, pkgs, unstable, ... }:
+{
+  nix = {
+    settings = {
+      experimental-features = [
+        "nix-command"
+        "flakes"
+      ];
+      auto-optimise-store = true;
+    };
+    gc = {
+      automatic = true;
+      dates = "weekly";
+      options = "--delete-older-than 7d";
+    };
+  };
+
+  _module.args.pkgsUnstable = import unstable {
+    inherit (pkgs.stdenv.hostPlatform) system;
+    inherit (config.nixpkgs) config;
+  };
+}