Add garabage collection

systems/modules/garbage-collection.nix

@@ -0,0 +1,12 @@
+{ config, pkgs, ... }:
+
+{
+  nix = {
+    settings.auto-optimise-store = true;
+    gc = {
+      automatic = true;
+      dates = "weekly";
+      options = "--delete-older-than 7d";
+    };
+  };
+}