Fix fromCompletableFuture cancelation leak

[armanbilge]

I inadvertently took over @TimWSpence's PR #3417 😅

[armanbilge]

Damn it. You broke it, you bought it 😩

[armanbilge]

Welp. We must have some global fiber monitor state somewhere, that persists across isolated runtimes ...

Edit oh lol

cats-effect/core/jvm-native/src/main/scala/cats/effect/unsafe/FiberMonitor.scala

Lines 219 to 228 in 204fca2

	private[FiberMonitor] final val BagReferences
	: ConcurrentLinkedQueue[WeakReference[WeakBag[Runnable]]] =
	new ConcurrentLinkedQueue()
	private[FiberMonitor] final val Bags: ThreadLocal[WeakBag[Runnable]] =
	ThreadLocal.withInitial { () =>
	val bag = new WeakBag[Runnable]()
	BagReferences.offer(new WeakReference(bag))
	bag
	}