Observers get a `'static` `Trigger` and that's unsound

## Bevy version

`0.14.1`

## What you did

I stored a `Trigger<'static, E, ()>` in a `Local` (could also be a `static`, a component, or anything else that can escape the observer system), and then I read it after the end of the first system call. 

```rs
use bevy_ecs::event::Event;
use bevy_ecs::observer::Trigger;
use bevy_ecs::system::{In, Local};
use bevy_ecs::world::World;

#[derive(Debug, Event)]
struct E(String);

fn observer(
    In(trigger): In<Trigger<'static, E, ()>>,
    mut last_trigger: Local<Option<Trigger<E, ()>>>,
) {
    println!("Now: {:?}", trigger.event());

    if let Some(last_trigger) = &*last_trigger {
        println!("Before: {:?}", last_trigger.event());
    }

    *last_trigger = Some(trigger);
}

fn main() {
    let mut world = World::new();
    world.observe(observer);
    world.flush();
    world.trigger(E("foo".to_string()));
    world.trigger(E("bar".to_string()));
}
```

## What went wrong

On my system it prints the following:

```
Now: E("foo")
Now: E("bar")
Before: E("bar")
```

Before should print `E("foo")`, not `E("bar")`, showing that it is actually accessing the same memory location as the new event.

MIRI also reports UB:

```
error: Undefined Behavior: constructing invalid value: encountered a dangling reference (use-after-free)
  --> E:\Programmi\Rust\.cargo\registry\src\index.crates.io-6f17d22bba15001f\bevy_ecs-0.14.1\src\observer\mod.rs:42:9
   |
42 |         self.event
   |         ^^^^^^^^^^ constructing invalid value: encountered a dangling reference (use-after-free)
   |
   = help: this indicates a bug in the program: it performed an invalid operation, and caused Undefined Behavior
   = help: see https://doc.rust-lang.org/nightly/reference/behavior-considered-undefined.html for further information
   = note: BACKTRACE:
   = note: inside `bevy_ecs::observer::Trigger::<'_, E>::event` at E:\Programmi\Rust\.cargo\registry\src\index.crates.io-6f17d22bba15001f\bevy_ecs-0.14.1\src\observer\mod.rs:42:9: 42:19
note: inside `observer`
  --> src/main.rs:16:34
   |
16 |         println!("Before: {:?}", last_trigger.event());
   |                                  ^^^^^^^^^^^^^^^^^^^^
   = note: inside `<for<'a, 'b> fn(bevy_ecs::system::In<bevy_ecs::observer::Trigger<'static, E>>, bevy_ecs::system::Local<'a, std::option::Option<bevy_ecs::observer::Trigger<'b, E>>>) {observer} as std::ops::FnMut<(bevy_ecs::system::In<bevy_ecs::observer::Trigger<'_, E>>, bevy_ecs::system::Local<'_, std::option::Option<bevy_ecs::observer::Trigger<'_, E>>>)>>::call_mut - shim(for<'a, 'b> fn(bevy_ecs::system::In<bevy_ecs::observer::Trigger<'static, E>>, bevy_ecs::system::Local<'a, std::option::Option<bevy_ecs::observer::Trigger<'b, E>>>) {observer})` at E:\Programmi\Rust\.rustup\toolchains\nightly-x86_64-pc-windows-msvc\lib\rustlib\src\rust\library\core\src\ops\function.rs:166:5: 166:75
   = note: inside `std::ops::function::impls::<impl std::ops::FnMut<(bevy_ecs::system::In<bevy_ecs::observer::Trigger<'_, E>>, bevy_ecs::system::Local<'_, std::option::Option<bevy_ecs::observer::Trigger<'_, E>>>)> for &mut for<'a, 'b> fn(bevy_ecs::system::In<bevy_ecs::observer::Trigger<'static, E>>, bevy_ecs::system::Local<'a, std::option::Option<bevy_ecs::observer::Trigger<'b, E>>>) {observer}>::call_mut` at E:\Programmi\Rust\.rustup\toolchains\nightly-x86_64-pc-windows-msvc\lib\rustlib\src\rust\library\core\src\ops\function.rs:294:13: 294:35
   = note: inside `<Func as bevy_ecs::system::SystemParamFunction<fn(bevy_ecs::system::In<Input>, F0) -> Out>>::run::call_inner::<bevy_ecs::observer::Trigger<'_, E>, (), bevy_ecs::system::Local<'_, std::option::Option<bevy_ecs::observer::Trigger<'_, E>>>, &mut for<'a, 'b> fn(bevy_ecs::system::In<bevy_ecs::observer::Trigger<'static, E>>, bevy_ecs::system::Local<'a, std::option::Option<bevy_ecs::observer::Trigger<'b, E>>>) {observer}>` at E:\Programmi\Rust\.cargo\registry\src\index.crates.io-6f17d22bba15001f\bevy_ecs-0.14.1\src\system\function_system.rs:735:21: 735:42
   = note: inside `<for<'a, 'b> fn(bevy_ecs::system::In<bevy_ecs::observer::Trigger<'static, E>>, bevy_ecs::system::Local<'a, std::option::Option<bevy_ecs::observer::Trigger<'b, E>>>) {observer} as bevy_ecs::system::SystemParamFunction<fn(bevy_ecs::system::In<bevy_ecs::observer::Trigger<'_, E>>, bevy_ecs::system::Local<'_, std::option::Option<bevy_ecs::observer::Trigger<'_, E>>>)>>::run` at E:\Programmi\Rust\.cargo\registry\src\index.crates.io-6f17d22bba15001f\bevy_ecs-0.14.1\src\system\function_system.rs:738:17: 738:57
   = note: inside `<bevy_ecs::system::FunctionSystem<fn(bevy_ecs::system::In<bevy_ecs::observer::Trigger<'_, E>>, bevy_ecs::system::Local<'_, std::option::Option<bevy_ecs::observer::Trigger<'_, E>>>), for<'a, 'b> fn(bevy_ecs::system::In<bevy_ecs::observer::Trigger<'static, E>>, bevy_ecs::system::Local<'a, std::option::Option<bevy_ecs::observer::Trigger<'b, E>>>) {observer}> as bevy_ecs::system::System>::run_unsafe` at E:\Programmi\Rust\.cargo\registry\src\index.crates.io-6f17d22bba15001f\bevy_ecs-0.14.1\src\system\function_system.rs:534:19: 534:47
   = note: inside `bevy_ecs::observer::runner::observer_system_runner::<E, ()>` at E:\Programmi\Rust\.cargo\registry\src\index.crates.io-6f17d22bba15001f\bevy_ecs-0.14.1\src\observer\runner.rs:409:9: 409:42
   = note: inside closure at E:\Programmi\Rust\.cargo\registry\src\index.crates.io-6f17d22bba15001f\bevy_ecs-0.14.1\src\observer\mod.rs:192:13: 200:14
   = note: inside `std::ops::function::impls::<impl std::ops::FnMut<((&bevy_ecs::entity::Entity, &for<'a, 'b> fn(bevy_ecs::world::DeferredWorld<'a>, bevy_ecs::observer::ObserverTrigger, bevy_ecs::bevy_ptr::PtrMut<'b>)),)> for &mut {closure@bevy_ecs::observer::Observers::invoke<E, std::array::IntoIter<bevy_ecs::component::ComponentId, 0>>::{closure#0}}>::call_mut` at E:\Programmi\Rust\.rustup\toolchains\nightly-x86_64-pc-windows-msvc\lib\rustlib\src\rust\library\core\src\ops\function.rs:294:13: 294:35
   = note: inside closure at E:\Programmi\Rust\.rustup\toolchains\nightly-x86_64-pc-windows-msvc\lib\rustlib\src\rust\library\core\src\iter\traits\iterator.rs:815:29: 815:36
   = note: inside closure at E:\Programmi\Rust\.cargo\registry\src\index.crates.io-6f17d22bba15001f\hashbrown-0.14.5\src\map.rs:4749:13: 4749:27
   = note: inside `hashbrown::raw::inner::RawIterRange::<(bevy_ecs::entity::Entity, for<'a, 'b> fn(bevy_ecs::world::DeferredWorld<'a>, bevy_ecs::observer::ObserverTrigger, bevy_ecs::bevy_ptr::PtrMut<'b>))>::fold_impl::<{closure@<hashbrown::map::Iter<'_, bevy_ecs::entity::Entity, for<'a, 'b> fn(bevy_ecs::world::DeferredWorld<'a>, bevy_ecs::observer::ObserverTrigger, bevy_ecs::bevy_ptr::PtrMut<'b>)> as std::iter::Iterator>::fold<(), {closure@std::iter::Iterator::for_each::call<(&bevy_ecs::entity::Entity, &for<'a, 'b> fn(bevy_ecs::world::DeferredWorld<'a>, bevy_ecs::observer::ObserverTrigger, bevy_ecs::bevy_ptr::PtrMut<'b>)), &mut {closure@bevy_ecs::observer::Observers::invoke<E, std::array::IntoIter<bevy_ecs::component::ComponentId, 0>>::{closure#0}}>::{closure#0}}>::{closure#0}}, ()>` at E:\Programmi\Rust\.cargo\registry\src\index.crates.io-6f17d22bba15001f\hashbrown-0.14.5\src\raw\mod.rs:3880:23: 3880:37
   = note: inside `<hashbrown::raw::inner::RawIter<(bevy_ecs::entity::Entity, for<'a, 'b> fn(bevy_ecs::world::DeferredWorld<'a>, bevy_ecs::observer::ObserverTrigger, bevy_ecs::bevy_ptr::PtrMut<'b>))> as std::iter::Iterator>::fold::<(), {closure@<hashbrown::map::Iter<'_, bevy_ecs::entity::Entity, for<'a, 'b> fn(bevy_ecs::world::DeferredWorld<'a>, bevy_ecs::observer::ObserverTrigger, bevy_ecs::bevy_ptr::PtrMut<'b>)> as std::iter::Iterator>::fold<(), {closure@std::iter::Iterator::for_each::call<(&bevy_ecs::entity::Entity, &for<'a, 'b> fn(bevy_ecs::world::DeferredWorld<'a>, bevy_ecs::observer::ObserverTrigger, bevy_ecs::bevy_ptr::PtrMut<'b>)), &mut {closure@bevy_ecs::observer::Observers::invoke<E, std::array::IntoIter<bevy_ecs::component::ComponentId, 0>>::{closure#0}}>::{closure#0}}>::{closure#0}}>` at E:\Programmi\Rust\.cargo\registry\src\index.crates.io-6f17d22bba15001f\hashbrown-0.14.5\src\raw\mod.rs:4151:18: 4151:58
   = note: inside `<hashbrown::map::Iter<'_, bevy_ecs::entity::Entity, for<'a, 'b> fn(bevy_ecs::world::DeferredWorld<'a>, bevy_ecs::observer::ObserverTrigger, bevy_ecs::bevy_ptr::PtrMut<'b>)> as std::iter::Iterator>::fold::<(), {closure@std::iter::Iterator::for_each::call<(&bevy_ecs::entity::Entity, &for<'a, 'b> fn(bevy_ecs::world::DeferredWorld<'a>, bevy_ecs::observer::ObserverTrigger, bevy_ecs::bevy_ptr::PtrMut<'b>)), &mut {closure@bevy_ecs::observer::Observers::invoke<E, std::array::IntoIter<bevy_ecs::component::ComponentId, 0>>::{closure#0}}>::{closure#0}}>` at E:\Programmi\Rust\.cargo\registry\src\index.crates.io-6f17d22bba15001f\hashbrown-0.14.5\src\map.rs:4747:9: 4750:11
   = note: inside `<hashbrown::map::Iter<'_, bevy_ecs::entity::Entity, for<'a, 'b> fn(bevy_ecs::world::DeferredWorld<'a>, bevy_ecs::observer::ObserverTrigger, bevy_ecs::bevy_ptr::PtrMut<'b>)> as std::iter::Iterator>::for_each::<&mut {closure@bevy_ecs::observer::Observers::invoke<E, std::array::IntoIter<bevy_ecs::component::ComponentId, 0>>::{closure#0}}>` at E:\Programmi\Rust\.rustup\toolchains\nightly-x86_64-pc-windows-msvc\lib\rustlib\src\rust\library\core\src\iter\traits\iterator.rs:818:9: 818:31
   = note: inside `bevy_ecs::observer::Observers::invoke::<E, std::array::IntoIter<bevy_ecs::component::ComponentId, 0>>` at E:\Programmi\Rust\.cargo\registry\src\index.crates.io-6f17d22bba15001f\bevy_ecs-0.14.1\src\observer\mod.rs:204:9: 204:61
   = note: inside `bevy_ecs::world::DeferredWorld::<'_>::trigger_observers_with_data::<E, std::array::IntoIter<bevy_ecs::component::ComponentId, 0>>` at E:\Programmi\Rust\.cargo\registry\src\index.crates.io-6f17d22bba15001f\bevy_ecs-0.14.1\src\world\deferred_world.rs:368:9: 368:76
   = note: inside `bevy_ecs::observer::trigger_event::trigger_event::<E, ()>` at E:\Programmi\Rust\.cargo\registry\src\index.crates.io-6f17d22bba15001f\bevy_ecs-0.14.1\src\observer\trigger_event.rs:61:13: 66:14
   = note: inside `<bevy_ecs::observer::TriggerEvent<E> as bevy_ecs::world::Command>::apply` at E:\Programmi\Rust\.cargo\registry\src\index.crates.io-6f17d22bba15001f\bevy_ecs-0.14.1\src\observer\trigger_event.rs:20:9: 20:72
   = note: inside `bevy_ecs::observer::<impl bevy_ecs::world::World>::trigger::<E>` at E:\Programmi\Rust\.cargo\registry\src\index.crates.io-6f17d22bba15001f\bevy_ecs-0.14.1\src\observer\mod.rs:275:9: 275:56
note: inside `main`
  --> src/main.rs:27:5
   |
27 |     world.trigger(E("bar".to_string()));
   |     ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^

note: some details are omitted, run with `MIRIFLAGS=-Zmiri-backtrace=full` for a verbose backtrace
```


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Observers get a `'static` `Trigger` and that's unsound #14924

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Observers get a 'static Trigger and that's unsound #14924

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Observers get a `'static` `Trigger` and that's unsound #14924