Corrects the cancellation mechanism of workflows from activities

[gvkries]

Fixes #17673

/cc @deanmarcussen

[deanmarcussen]

Yes, I'm not convinced using a CancellationToken here was the best idea, but this fixes it, without changing the api much, which is probably for the best.

cancellationToken.ThrowIfCancellationRequested() which is pretty standard usage, later down in an activities usage, will trun it from Aborted to Faulted. But as I said, fixes it without changing the surface of things too much, so LGTM.

Thanks!