Job Failed #67299002: FileExistsError

Job #67299002 failed for 2f8082ed

    An unhandled exception occured:
    
    Traceback (most recent call last):
      File "/builds/BuildStream/buildstream/dist/buildstream/buildstream/_scheduler/job.py", line 320, in _child_action
        result = self._action_cb(element)
      File "/builds/BuildStream/buildstream/dist/buildstream/buildstream/_scheduler/buildqueue.py", line 34, in process
        element._assemble()
      File "/builds/BuildStream/buildstream/dist/buildstream/buildstream/element.py", line 1448, in _assemble
        collect = self.assemble(sandbox)
      File "/builds/BuildStream/buildstream/dist/buildstream/buildstream/scriptelement.py", line 280, in assemble
        SandboxFlags.ROOT_READ_ONLY if self.__root_read_only else 0)
      File "/builds/BuildStream/buildstream/dist/buildstream/buildstream/sandbox/_sandboxbwrap.py", line 76, in run
        mount_map = MountMap(self, flags & SandboxFlags.ROOT_READ_ONLY)
      File "/builds/BuildStream/buildstream/dist/buildstream/buildstream/sandbox/_mount.py", line 106, in __init__
        self.mounts['/'] = Mount(sandbox, '/', not root_readonly)
      File "/builds/BuildStream/buildstream/dist/buildstream/buildstream/sandbox/_mount.py", line 66, in __init__
        os.makedirs(self.mount_source, exist_ok=True)
      File "/usr/lib/python3.5/os.py", line 241, in makedirs
        mkdir(name, mode)
    FileExistsError: [Errno 17] File exists: '/builds/BuildStream/buildstream/dist/buildstream/tmp/test_stack0/cache/build/stack-another-hi-ii7pw49p/scratch/_/mount'

Related to !452 (merged).