nixpart: Update to bugfix version 0.4.1.

Contains the following fix:

  - Fix mounting btrfs when mount_only (-m) is used.

I would like to update blivet as well, but at the moment it breaks for
nixpart, so let's retry later when we're at 0.5.x or even 1.x :-)

Signed-off-by: aszlig <aszlig@redmoonstudios.org>