Fix accidentally removed skipped_system insertion (thanks @chescock)

Author: alice-i-cecile

crates/bevy_ecs/src/schedule/executor/multi_threaded.rs

@@ -595,6 +595,7 @@ impl ExecutorState {
                 }
                 ValidationOutcome::Skipped => false,
             };
+            self.skipped_systems.insert(system_index);
 
             should_run &= valid_params;
         }