Fix wrong type to link_output

Fix wrong type to link_output

$ sudo mkosi --default /dev/null -d fedora -r 29 -t directory -p dnf
...
Traceback (most recent call last):
  File "/bin/mkosi", line 3959, in <module>
    main()
  File "/bin/mkosi", line 3949, in main
    build_stuff(args)
  File "/bin/mkosi", line 3809, in build_stuff
    link_output(args, workspace, raw or tar)
  File "/bin/mkosi", line 2478, in link_output
    os.rename(os.path.join(workspace, "root"), args.output)
  File "/usr/lib64/python3.7/posixpath.py", line 80, in join
    a = os.fspath(a)
TypeError: expected str, bytes or os.PathLike object, not TemporaryDirectory