Deterministic fallible_systems example (#17813)

# Objective

- `fallible_systems` example is not deterministic

## Solution

- Make it deterministic
- Also fix required feature declaration

Cargo.toml

@@ -2156,13 +2156,13 @@ wasm = false
 name = "fallible_systems"
 path = "examples/ecs/fallible_systems.rs"
 doc-scrape-examples = true
+required-features = ["bevy_mesh_picking_backend"]
 
 [package.metadata.example.fallible_systems]
 name = "Fallible Systems"
 description = "Systems that return results to handle errors"
 category = "ECS (Entity Component System)"
 wasm = false
-required-features = ["bevy_mesh_picking_backend"]
 
 [[example]]
 name = "startup_system"
@@ -4000,7 +4000,6 @@ name = "Sprite Picking"
 description = "Demonstrates picking sprites and sprite atlases"
 category = "Picking"
 wasm = true
-required-features = ["bevy_sprite_picking_backend"]
 
 [[example]]
 name = "debug_picking"

examples/ecs/fallible_systems.rs

@@ -6,6 +6,8 @@ use bevy::math::sampling::UniformMeshSampler;
 use bevy::prelude::*;
 
 use rand::distributions::Distribution;
+use rand::SeedableRng;
+use rand_chacha::ChaCha8Rng;
 
 fn main() {
     let mut app = App::new();
@@ -68,7 +70,7 @@ fn setup(
     mut meshes: ResMut<Assets<Mesh>>,
     mut materials: ResMut<Assets<StandardMaterial>>,
 ) -> Result {
-    let mut rng = rand::thread_rng();
+    let mut seeded_rng = ChaCha8Rng::seed_from_u64(19878367467712);
 
     // Make a plane for establishing space.
     commands.spawn((
@@ -116,7 +118,7 @@ fn setup(
     });
 
     // Add sample points as children of the sphere:
-    for point in distribution.sample_iter(&mut rng).take(10000) {
+    for point in distribution.sample_iter(&mut seeded_rng).take(10000) {
         sphere.with_child((
             Mesh3d(point_mesh.clone()),
             MeshMaterial3d(point_material.clone()),