x/sync: Use WithCancelCause for errgroup.WithContext

[jonjohnsonjr]

When using errgroup.WithContext, the first non-nil error encountered will cancel the context and then be returned by the next call to Wait. This works well when you actually call Wait, but if that canceled context prevents you from calling Wait, you just get an unhelpful context canceled error.

I believe the new context.WithCancelCause added in 1.20 would fix this, so I propose we use that instead of this context.WithCancel so we can propagate the error here, here, and here.

[gopherbot]

Change https://go.dev/cl/481255 mentions this issue: errgroup: use WithCancelCause to cancel context

[rsc]

Seems to have landed without a proposal acceptance but too late to do anything about that now.