ostree mirror fallback breaks assertion

Summary

When trying to fetch from ostree mirror when upstream is down, fetchqueue fails with a broken assertion.

Steps to reproduce

• Have a setup with an upstream and a mirror ostree repositories.
• Take down upstream.
• Fetch.

What is the current bug behavior?

        Traceback (most recent call last):
          File "/home/valentin/repos/buildstream/buildstream/_scheduler/queues/queue.py", line 310, in _job_done
            processed = self.done(job, element, result, success)
          File "/home/valentin/repos/buildstream/buildstream/_scheduler/queues/fetchqueue.py", line 78, in done
            assert element._get_consistency() == Consistency.CACHED
        AssertionError

What is the expected correct behavior?

No error.

Relevant logs and/or screenshots

Possible fixes

Other relevant information

• BuildStream version affected: /milestone %BuildStream_v1.2

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