Extract configuration to own module

modules/nix-config.nix

@@ -0,0 +1,22 @@
+{ config, lib, pkgs, ... }:
+
+{
+  nix = {
+    package = pkgs.nixUnstable;
+    gc = {
+      automatic = true;
+      dates = "weekly";
+      options = "--delete-older-than 30d";
+    };
+
+    settings = {
+      auto-optimise-store = true;
+      experimental-features = [ "nix-command" "flakes" "repl-flake" ];
+      warn-dirty = false;
+
+      # avoid unwanted garbage collection when using direnv
+      keep-outputs = true;
+      keep-derivations = true;
+    };
+  };
+}