Fix custom_loop example to include plugin finalization

[philpax]

Objective

The custom_loop example didn't replicate the app.finish / app.cleanup calls from the default runner; I discovered this when trying to troubleshoot why my application with a custom loop wasn't calling its plugin finalizers, and realised that the upstream example that I'd referenced didn't have the relevant calls.

Solution

Added the missing calls, replicating what the default runner does:

bevy/crates/bevy_app/src/app.rs

Lines 895 to 896 in d390420

	app.finish();
	app.cleanup();

Testing

I've confirmed that adding these two calls to my application fixed the issue I was encountering. I haven't tested it within the example itself as it's relatively straightforward and I didn't want to pollute the example with a plugin using a finalizer.

[github-actions]

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