Auto garbage collect

configuration.nix

@@ -51,4 +51,8 @@ in {
 
   system.autoUpgrade.enable = true;
   system.stateVersion = "18.09";
+
+  nix.gc.automatic = true;
+  nix.gc.dates = "weekly";
+  nix.gc.options = "--delete-older-than 30d";
 }