Fix UB detected by Miri when running the multi-threaded executor

[chescock]

Objective

Fix UB detected by Miri when running the multi-threaded executor. It intermittently fails with:

error: Undefined Behavior: not granting access to tag <5526305> because that would remove [SharedReadOnly for <5518830>] which is strongly protected
   --> crates\bevy_ecs\src\schedule\executor\multi_threaded.rs:785:36
    |
785 |         let system = &mut unsafe { &mut *systems[system_index].get() }.system;
    |                                    ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^ Undefined Behavior occurred here
    |
    = help: this indicates a potential bug in the program: it performed an invalid operation, but the Stacked Borrows rules it violated are still experimental
    = help: see https://github.com/rust-lang/unsafe-code-guidelines/blob/master/wip/stacked-borrows.md for further information
help: <5526305> was created by a SharedReadWrite retag at offsets [0x0..0x80]
   --> crates\bevy_platform\src\cell\sync_unsafe_cell.rs:58:8
help: <5518830> is this argument
   --> crates\bevy_ecs\src\schedule\executor\multi_threaded.rs:334:9
    |
334 |         system: &ScheduleSystem,
    |         ^^^^^^
    = note: this is on thread `schedule::executor::multi_threaded::tests::check_spawn_exclusive_system_task_miri`
    = note: stack backtrace:

The issue is that system_completed() calls self.tick_executor();, which spawns new tasks. It may spawn a task to execute apply_deferred, which will create a &mut to systems with unapplied buffers. If that task is scheduled before system_completed() completes, then that &mut will invalidate the &ScheduleSystem parameter while it's strongly protected.

Solution

Pass the &SyncUnsafeCell<SystemWithAccess> around directly, and only convert it to a &mut SystemWithAccess when it is used. This ensures that all references have ended before the call to tick_executor(). The conversion just reinterprets the pointers and has no runtime effect, so doing it at different times should not affect performance.