avoid starpu internals use in scheduler

src/sched_policies/parallel_heft.c

@@ -489,7 +489,7 @@ static int _parallel_heft_push_task(struct starpu_task *task, unsigned prio, uns
 	}
 
 	//_STARPU_DEBUG("Scheduler parallel heft: kernel (%u)\n", nimpl_best);
-	_starpu_get_job_associated_to_task(task)->nimpl = nimpl_best;
+	starpu_task_set_implementation(task, nimpl_best);
 	/* we should now have the best worker in variable "best" */
 	_STARPU_TASK_BREAK_ON(task, sched);
 	return push_task_on_best_worker(task, best, best_exp_end, prio, sched_ctx_id);