disk image code refactor breaks cheribsd builds?

[nwf]

With aba62a2 in tree, attempting to make disk images fails with

Traceback (most recent call last):
  File "/home/nwf20/cheri/mainline/cheribuild/cheribuild.py", line 42, in <module>
    main()
  File "/local/scratch/nwf20/cheri/mainline/cheribuild/pycheribuild/__main__.py", line 227, in main
    real_main()
  File "/local/scratch/nwf20/cheri/mainline/cheribuild/pycheribuild/__main__.py", line 219, in real_main
    targetManager.run(cheriConfig)
  File "/local/scratch/nwf20/cheri/mainline/cheribuild/pycheribuild/targets.py", line 320, in run
    target.execute(config)
  File "/local/scratch/nwf20/cheri/mainline/cheribuild/pycheribuild/targets.py", line 92, in execute
    project.process()
  File "/local/scratch/nwf20/cheri/mainline/cheribuild/pycheribuild/projects/disk_image.py", line 473, in process
    self.__process()
  File "/local/scratch/nwf20/cheri/mainline/cheribuild/pycheribuild/projects/disk_image.py", line 535, in __process
    pathInImage = p.relative_to(self.extraFilesDir)
  File "/usr/lib/python3.6/pathlib.py", line 874, in relative_to
    .format(str(self), str(formatted)))
ValueError: '/home/nwf20/cheri/mainline/host-output/kyua-cheribsd-pkgcache/usr/lib/libarchive.so.6' does not start with '/home/nwf20/cheribsd-extra-files'

I'm invoking

cheribuild.py --disk-image/extra-files=${HOME}/cheribsd-extra-files --disk-image-path=... --disk-image/auto-add-files