Code review comment for ~paride/curtin:target_arch

Also tested:

 - test_old_apt_features.py on amd64 (succeeds)
 - test_old_apt_features.py on ppc64el (fails, but fails
   in the exact same way without this branch applied,
   and I don't think the fix belongs here)

Also tested that the i386 image is not downloaded by vmtest-sync-images on ppc64el. This happened before because of the tests setting 'arch = i386'.