Move scene spawner systems to SpawnScene schedule (#9260)

# Objective

- Fixes https://github.com/bevyengine/bevy/issues/9250

## Changelog

- Move scene spawner systems to a new SpawnScene schedule which is after
Update and before PostUpdate (schedule order:
[PreUpdate][Update][SpawnScene][PostUpdate])

## Migration Guide

- Move scene spawner systems to a new SpawnScene schedule which is after
Update and before PostUpdate (schedule order:
[PreUpdate][Update][SpawnScene][PostUpdate]), you might remove system
ordering code related to scene spawning as the execution order has been
guaranteed by bevy engine.

---------

Co-authored-by: Hennadii Chernyshchyk <genaloner@gmail.com>

crates/bevy_app/src/main_schedule.rs

@@ -79,6 +79,11 @@ pub struct FixedUpdate;
 #[derive(ScheduleLabel, Clone, Debug, PartialEq, Eq, Hash)]
 pub struct Update;
 
+/// The schedule that contains scene spawning.
+/// This is run by the [`Main`] schedule.
+#[derive(ScheduleLabel, Clone, Debug, PartialEq, Eq, Hash)]
+pub struct SpawnScene;
+
 /// The schedule that contains logic that must run after [`Update`]. For example, synchronizing "local transforms" in a hierarchy
 /// to "global" absolute transforms. This enables the [`PostUpdate`] transform-sync system to react to "local transform" changes in
 /// [`Update`] without the [`Update`] systems needing to know about (or add scheduler dependencies for) the "global transform sync system".
@@ -112,6 +117,7 @@ impl Default for MainScheduleOrder {
                 Box::new(StateTransition),
                 Box::new(RunFixedUpdateLoop),
                 Box::new(Update),
+                Box::new(SpawnScene),
                 Box::new(PostUpdate),
                 Box::new(Last),
             ],

crates/bevy_scene/src/lib.rs

@@ -11,6 +11,7 @@ mod scene_spawner;
 #[cfg(feature = "serialize")]
 pub mod serde;
 
+use bevy_ecs::schedule::IntoSystemConfigs;
 pub use bundle::*;
 pub use dynamic_scene::*;
 pub use dynamic_scene_builder::*;
@@ -27,7 +28,7 @@ pub mod prelude {
     };
 }
 
-use bevy_app::prelude::*;
+use bevy_app::{prelude::*, SpawnScene};
 use bevy_asset::AddAsset;
 
 #[derive(Default)]
@@ -40,9 +41,7 @@ impl Plugin for ScenePlugin {
             .add_asset::<Scene>()
             .init_asset_loader::<SceneLoader>()
             .init_resource::<SceneSpawner>()
-            .add_systems(Update, scene_spawner_system)
-            // Systems `*_bundle_spawner` must run before `scene_spawner_system`
-            .add_systems(PreUpdate, scene_spawner);
+            .add_systems(SpawnScene, (scene_spawner, scene_spawner_system).chain());
     }
 }