Don't set up more than 1 abort signal listener

[gustavohenke]

🤦 Rxjs' fromEvent creates a new listener on every subscription, but this is a bit hidden because the abort signal isn't subscribed to directly:

concurrently/src/completion-listener.ts

Lines 96 to 109 in a7a5894

	const abort =
	abortSignal &&
	Rx.fromEvent(abortSignal, 'abort', { once: true }).pipe(
	// The abort signal must happen before commands are killed, otherwise new commands
	// might spawn. Because of this, it's not be possible to capture the close events
	// without an immediate delay
	delay(0, this.scheduler),
	map(() => undefined),
	);
	const closeStreams = commands.map((command) =>
	abort
	? // Commands that have been started must close.
	Rx.race(command.close, abort.pipe(filter(() => command.state === 'stopped')))

Sounds like just sharing the underlying source observable is enough.

Fixes #502

[coveralls]

coverage: 98.263%. remained the same
when pulling 74cc2f2 on abortsignal-leak
into a7a5894 on main.