An entity swapping contexts fails if running in FixedUpdate schedule

## Problem
When changing the `RapierContextEntityLink` component of an entity, I expect it to properly become a part of the new world it was added to. This works correctly if the PhysicsPlugin is being run in the `Update` schedule, but fails if it is being run in the `FixedUpdate` schedule.

### Reproducible using this code:
https://github.com/AnthonyTornetta/bevy_rapier/blob/master/bevy_rapier3d/examples/change_contexts3.rs

You can test the success case by removing `.in_fixed_schedule()` on the plugin's initialization. In the failure: as soon as it changes worlds it is not successfully added to the new world, so it just disappears.

I will do some poking around and see if I can fix this.