Don't load cargo shuttle by default (#9194)

* Don't load cargo shuttle by default

Realistically only I am going to care and it wastes a few seconds per test / debugging locally.

* Make it a general cvar instead

* a

Content.Server/Cargo/Systems/CargoSystem.cs

@@ -27,7 +27,8 @@ public sealed partial class CargoSystem : SharedCargoSystem
     public override void Shutdown()
     {
         base.Shutdown();
-        Cleanup();
+        ShutdownShuttle();
+        CleanupShuttle();
     }
 
     private void OnStationInit(StationInitializedEvent ev)