Fix race in 'executeOperation(props:operation)'

[mchakravarty]

If the task got cancelled while still waiting for dependencies, 'completePendingTask(with:)' was not executed, leaving a zombie in the queue.

The fix is to move the defer clause to the very beginning of the function.

[mattmassicotte]

Ugh wow, great find. I hope this didn't cause you too much trouble.

I'd like to add a test to expose this problem, but that's non-critical.

(also, safe to ignore that linux CI failure, known issue and I need to resolve it anyways)