fn is_cancelled(&self) -> bool {
        if let Some(backoff_and_cancellable) = &self.backoff_and_cancellable {
            return backoff_and_cancellable.cancellation_token.is_cancelled();
        }
        false
    }
Checks if the task is cancelled, if available.
prover/crates/lib/prover_job_processor/src/task_wiring/job_picker_task.rs