element.py: Pass bare_directory also to SandboxRemote()

This fixes the following unhandled exception with remote execution:

    Traceback (most recent call last):
      File "buildstream/buildstream/_scheduler/jobs/job.py", line 417, in _child_action
        result = self.child_process()  # pylint: disable=assignment-from-no-return
      File "buildstream/buildstream/_scheduler/jobs/elementjob.py", line 94, in child_process
        return self._action_cb(self._element)
      File "buildstream/buildstream/_scheduler/queues/buildqueue.py", line 71, in process
        return element._assemble()
      File "buildstream/buildstream/element.py", line 1556, in _assemble
        self.__sandbox(rootdir, output_file, output_file, self.__sandbox_config) as sandbox:  # nopep8
      File "/usr/lib/python3.7/contextlib.py", line 112, in __enter__
        return next(self.gen)
      File "buildstream/buildstream/element.py", line 2182, in __sandbox
        allow_real_directory=False)
      File "buildstream/buildstream/sandbox/_sandboxremote.py", line 45, in __init__
        super().__init__(*args, **kwargs)
      File "buildstream/buildstream/sandbox/sandbox.py", line 101, in __init__
        self.__bare_directory = kwargs['bare_directory']
    KeyError: 'bare_directory'

Fixes: 825963b5

approved this merge request

merged

mentioned in commit 82e971ef