Fix cnf clean

When the change was made to cache the bnd repo in cnf/cache, the clean
of cnf/cache was not fixed to avoid deleting the cnf/cache directory.
This problem was discovered in
https://github.com/bndtools/bndtools/issues/945.

Signed-off-by: BJ Hargrave <bj@bjhargrave.com>
Merge pull request #950 from bjhargrave/fix-cnf-clean

Fix cnf clean