worker: always create subtasks on the same worker

... to fix the following crash due to race between the automated task assignment
to workers by the hub and the `hub.worker.assign_task` XML-RPC method.

```
Traceback (most recent call last):
  File "/usr/lib/python3.9/site-packages/kobo/worker/taskmanager.py", line 423, in run_task
    task.run()
  File "/usr/lib/python3.9/site-packages/osh/worker/tasks/task_errata_diff_build.py", line 52, in run
    self.hub.worker.assign_task(subtask_id)
  File "/usr/lib64/python3.9/xmlrpc/client.py", line 1122, in __call__
    return self.__send(self.__name, args)
  File "/usr/lib64/python3.9/xmlrpc/client.py", line 1464, in __request
    response = self.__transport.request(
  File "/usr/lib/python3.9/site-packages/kobo/xmlrpc.py", line 598, in request
    result = transport_class.request(self, *args, **kwargs)
  File "/usr/lib64/python3.9/xmlrpc/client.py", line 1166, in request
    return self.single_request(host, handler, request_body, verbose)
  File "/usr/lib/python3.9/site-packages/kobo/xmlrpc.py", line 511, in _single_request3
    return self.parse_response(response)
  File "/usr/lib64/python3.9/xmlrpc/client.py", line 1354, in parse_response
    return u.close()
  File "/usr/lib64/python3.9/xmlrpc/client.py", line 668, in close
    raise Fault(**self._stack[0])
xmlrpc.client.Fault: <Fault 1: Traceback (most recent call last):
  File "/usr/lib/python3.6/site-packages/kobo/hub/models.py", line 861, in assign_task
    self.__lock(worker_id, new_state=TASK_STATES["ASSIGNED"], initial_states=(TASK_STATES["FREE"], TASK_STATES["CREATED"]))
  File "/usr/lib/python3.6/site-packages/kobo/hub/models.py", line 836, in __lock
    raise ObjectDoesNotExist()
django.core.exceptions.ObjectDoesNotExist
```

Related:  release-engineering/kobo#230
Resolves: openscanhub#156
Resolves: https://issues.redhat.com/browse/OSH-358

osh/worker/tasks/task_build.py

@@ -35,8 +35,7 @@ def run(self):
 
         # scan base
         if base_task_args:
-            subtask_id = self.spawn_subtask(*base_task_args)
-            self.hub.worker.assign_task(subtask_id)
+            self.spawn_subtask(*base_task_args, inherit_worker=True)
             self.wait()
 
         if upload_id:

osh/worker/tasks/task_errata_diff_build.py

@@ -40,17 +40,16 @@ def run(self):
         # update analyzers version cache if needed
         cache_task_args = self.hub.worker.ensure_cache(mock_config, scanning_session_id)
         if cache_task_args is not None:
-            cache_subtask_id = self.spawn_subtask(*cache_task_args)
-            self.hub.worker.assign_task(cache_subtask_id)
+            self.spawn_subtask(*cache_task_args, inherit_worker=True)
             self.wait()
 
         # (re)scan base if needed
         base_task_args = self.hub.worker.ensure_base_is_scanned_properly(scan_id, self.task_id)
         if base_task_args is not None:
             self.hub.worker.set_scan_to_basescanning(scan_id)
-            subtask_id = self.spawn_subtask(*base_task_args)
+
+            subtask_id = self.spawn_subtask(*base_task_args, inherit_worker=True)
             self.hub.worker.create_sb(subtask_id)
-            self.hub.worker.assign_task(subtask_id)
 
             self.wait()
             self.hub.worker.set_scan_to_scanning(scan_id)