Replace fixed timestep in alien_cake_addict example with timer (bev…

…yengine#5760)

# Objective

Examples should use the correct tools for the job.

## Solution

A fixed timestep, by design, can step multiple times consecutively in a single update.

That property used to crash the `alien_cake_addict` example (bevyengine#2525), which was "fixed" in bevyengine#3411 (by just not panicking). The proper fix is to use a timer instead, since the system is supposed to spawn a cake every 5 seconds. 

---

A timer guarantees a minimum duration. A fixed timestep guarantees a fixed number of steps per second.
Each one works by essentially sacrificing the other's guarantee.

You can use them together, but no other systems are timestep-based in this example, so the timer is enough.

examples/games/alien_cake_addict.rs

@@ -2,7 +2,7 @@
 
 use std::f32::consts::PI;
 
-use bevy::{ecs::schedule::SystemSet, prelude::*, time::FixedTimestep};
+use bevy::{ecs::schedule::SystemSet, prelude::*};
 use rand::Rng;
 
 #[derive(Clone, Eq, PartialEq, Debug, Hash)]
@@ -11,9 +11,13 @@ enum GameState {
     GameOver,
 }
 
+#[derive(Resource)]
+struct BonusSpawnTimer(Timer);
+
 fn main() {
     App::new()
         .init_resource::<Game>()
+        .insert_resource(BonusSpawnTimer(Timer::from_seconds(5.0, true)))
         .add_plugins(DefaultPlugins)
         .add_state(GameState::Playing)
         .add_startup_system(setup_cameras)
@@ -23,17 +27,13 @@ fn main() {
                 .with_system(move_player)
                 .with_system(focus_camera)
                 .with_system(rotate_bonus)
-                .with_system(scoreboard_system),
+                .with_system(scoreboard_system)
+                .with_system(spawn_bonus),
         )
         .add_system_set(SystemSet::on_exit(GameState::Playing).with_system(teardown))
         .add_system_set(SystemSet::on_enter(GameState::GameOver).with_system(display_score))
         .add_system_set(SystemSet::on_update(GameState::GameOver).with_system(gameover_keyboard))
         .add_system_set(SystemSet::on_exit(GameState::GameOver).with_system(teardown))
-        .add_system_set(
-            SystemSet::new()
-                .with_run_criteria(FixedTimestep::step(5.0))
-                .with_system(spawn_bonus),
-        )
         .add_system(bevy::window::close_on_esc)
         .run();
 }
@@ -291,13 +291,17 @@ fn focus_camera(
 
 // despawn the bonus if there is one, then spawn a new one at a random location
 fn spawn_bonus(
+    time: Res<Time>,
+    mut timer: ResMut<BonusSpawnTimer>,
     mut state: ResMut<State<GameState>>,
     mut commands: Commands,
     mut game: ResMut<Game>,
 ) {
-    if *state.current() != GameState::Playing {
+    // make sure we wait enough time before spawning the next cake
+    if !timer.0.tick(time.delta()).finished() {
         return;
     }
+
     if let Some(entity) = game.bonus.entity {
         game.score -= 3;
         commands.entity(entity).despawn_recursive();