restore proot for inventory updates

- in the past, inv. update jobs called `awx-manage inventory_update`
  which took care of setting up process isolation
- at this point, though, inv. update jobs call runner / ansible-inventory
  directly, so we need another way to put process isolation in place
- thankfully, there was already support for providing process isolation
  for other types of jobs (namely JT Jobs, Project Updates and Ad Hoc
  commands)
- so, we do what those other jobs do and override the stub for should_use_proot
  (which by default returns false) so that it keys off of the
  `AWX_PROOT_ENABLED` setting

awx/main/tasks.py

@@ -2461,6 +2461,12 @@ class RunInventoryUpdate(BaseTask):
     event_model = InventoryUpdateEvent
     event_data_key = 'inventory_update_id'
 
+    def should_use_proot(self, inventory_update):
+        '''
+        Return whether this task should use proot.
+        '''
+        return getattr(settings, 'AWX_PROOT_ENABLED', False)
+
     @property
     def proot_show_paths(self):
         return [settings.AWX_ANSIBLE_COLLECTIONS_PATHS]