Fix everything.

system/settings/nix/default.nix

@@ -0,0 +1,23 @@
+{ config, lib, pkgs, unstable, ... }:
+{
+  imports = [ ./gc ];
+
+  options.nixpkgs.allowUnfreePackages = lib.mkOption {
+    type = with lib.types; listOf str;
+  };
+
+  config = {
+    nix.settings = {
+      experimental-features = [
+        "nix-command"
+        "flakes"
+      ];
+      auto-optimise-store = true;
+    };
+
+    _module.args.pkgsUnstable = import unstable {
+      inherit (pkgs.stdenv.hostPlatform) system;
+      inherit (config.nixpkgs) config;
+    };
+  };
+}

system/settings/nix/gc/default.nix

@@ -0,0 +1,8 @@
+{ ... }:
+{
+  nix.gc = {
+    automatic = true;
+    dates = "weekly";
+    options = "--delete-older-than 7d";
+  };
+}