Fix one test

[majocha]

Description

error on one workflow should cancel all others test` became flaky after my changes.
This became very apparent when running tests in parallel in CI.

The mistake was to rely on Async.OnCancel to detect cancellation inside the computation. This is absolutely unreliable because the computation can check for cancellation before Async.OnCancel manages to register its callback. Even inside Async.OnCancel implementation, there's a bind that will check cancellation and prevent the callback from executing.

Checklist

• Ran locally a few thousand parallel iterations to make sure this is good.

[github-actions]

✅ No release notes required

[vzarytovskii]

^ story of my life

[majocha]

The failing test here is unrelated 😄 and depends on another fix here: #18074

[psfinaki]

Great, thanks :)