Use the regular build tag for kiwi container builds
    
    Container builds seem to fail when mock is run in "simple" isolation
    (ie. plain chroot) mode but work fine in "nspawn" isolation
    (i.e new chroot) mode. Use the regular build tag (which uses this mode)
    instead for container builds.
    
    Signed-off-by: Neal Gompa <ngompa@fedoraproject.org>