Systems could potentially get preempted by tasks from other worlds

## Bevy version
main

## What you did
while profiling ,I noticed that certain systems like `propagate_transforms `or `animate_targets `that utilize `par_iter `seemed to take an unusually long time to execute sometimes.


## Additional information
If the systems in the main world using `par_iter ` ,its own thread may potentially get occupied by other tasks.Specifically,if the thief happens to be a heavy task in the rendering world , systems in the main world will be blocked,the pipeline rendering is likely to degrade into a serial approach.