hub: `get_tasks_to_assign` does not respect channel selection for awaited tasks

[lzaoral]

The query for awaited tasks does not take the worker channel into account at all:

kobo/kobo/hub/xmlrpc/worker.py

Lines 194 to 197 in 86ee235

	# awaited tasks
	for task in Task.objects.free().filter(awaited=True).order_by("-priority", "id")[:max_tasks]:
	task_info = task.export(flat=False)
	task_list.append(task_info)

See https://github.com/release-engineering/kobo/pull/237/files/d4c514b1ed0b2862021b5c34cd14e5ab89cbded3#r1419709869 for details.