Setup regular garbage collection in common.nix

nixos/common.nix

@@ -0,0 +1,9 @@
+{ config, pkgs, ... }:
+
+{
+  nix.gc = {
+    automatic = true;
+    dates = "weekly";
+    options = "--delete-older-than 7d";
+  };
+}

nixos/vanguard-xenia.nix

@@ -1,7 +1,3 @@
-# Edit this configuration file to define what should be installed on
-# your system.  Help is available in the configuration.nix(5) man page
-# and in the NixOS manual (accessible by running ‘nixos-help’).
-
 { config, pkgs, ... }:
 
 let
@@ -11,6 +7,7 @@ in
 {
   imports =
     [
+      ./common.nix
       ./packages.nix
       (import "${home-manager}/nixos")
     ];