BUG when building in non-strict mode

[abderrahim]

Trying to build freedesktop-sdk in non-strict mode, I got the following message.

[--:--:--] BUG     [????????] freedesktop-sdk.bst:components/sqlite.bst: Build

    An unhandled exception occured:
    
    Traceback (most recent call last):
      File "/home/abderrahim/.virtualenvs/bst-master/lib/python3.9/site-packages/buildstream/_scheduler/jobs/job.py", line 442, in child_action
        result = self.child_process()  # pylint: disable=assignment-from-no-return
      File "/home/abderrahim/.virtualenvs/bst-master/lib/python3.9/site-packages/buildstream/_scheduler/jobs/elementjob.py", line 93, in child_process
        return self._action_cb(self._element)
      File "/home/abderrahim/.virtualenvs/bst-master/lib/python3.9/site-packages/buildstream/_scheduler/queues/buildqueue.py", line 58, in _assemble_element
        return element._assemble()
      File "/home/abderrahim/.virtualenvs/bst-master/lib/python3.9/site-packages/buildstream/element.py", line 1776, in _assemble
        return self._cache_artifact(sandbox, collect)
      File "/home/abderrahim/.virtualenvs/bst-master/lib/python3.9/site-packages/buildstream/element.py", line 1821, in _cache_artifact
        assert self.__cache_key is not None
    AssertionError

It isn't reproducible in that rerunning the pipeline succeeds (or actually fails in a subsequent element; this is the second time it happened).

This is using bc81cc6360f4777493d3bdcc3fa382bd5f0f358a (branch abderrahim/bst2)