[roles] No config change unless enabled

roles/printing/default.nix

@@ -2,18 +2,22 @@
 
 with lib;
 
+let
+  cfg = config.roles.printing;
+in
 {
   options.roles.printing = {
     enable = mkEnableOption "Enable default printing setup";
   };
 
-  config = {
-    services.printing.enable = true;
+  config = mkIf cfg.enable
+    {
+      services.printing.enable = true;
 
-    services.avahi = {
-      enable = true;
-      nssmdns4 = true;
-      openFirewall = true;
+      services.avahi = {
+        enable = true;
+        nssmdns4 = true;
+        openFirewall = true;
+      };
     };
-  };
 }