Replace fixed timestep in `alien_cake_addict` example with timer

[maniwani]

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 (#2525), which was "fixed" in #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.

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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.