Fix --track with local sources

This fixes the following assertion failure:

Unknown exception in SIGCHLD handler
Traceback (most recent call last):
  File "/usr/lib/python3.6/asyncio/unix_events.py", line 810, in _sig_chld
    self._do_waitpid_all()
  File "/usr/lib/python3.6/asyncio/unix_events.py", line 876, in _do_waitpid_all
    self._do_waitpid(pid)
  File "/usr/lib/python3.6/asyncio/unix_events.py", line 910, in _do_waitpid
    callback(pid, returncode, *args)
  File "buildstream/buildstream/_scheduler/job.py", line 327, in child_complete
    self.complete(self, returncode, element)
  File "buildstream/buildstream/_scheduler/queue.py", line 199, in job_done
    if self.done(element, job.result, returncode):
  File "buildstream/buildstream/_scheduler/fetchqueue.py", line 63, in done
    assert(element._consistency() == Consistency.CACHED)
AssertionError