chroot: do not use setgroups if it is blocked
chroot: do not use setgroups if it is blocked

if setgroups is blocked to set up the user namespace, do not attempt
to use it to clear the additional groups.

Signed-off-by: Giuseppe Scrivano <20002b7b24c75512093a71a58285826d5c504fca@redhat.com>

nalind/buildah,rhatdan/buildah,projectatomic/buildah,TomSweeneyRedHat/buildah,TomSweeneyRedHat/buildah,nalind/buildah-1,TomSweeneyRedHat/buildah,nalind/buildah-1,nalind/buildah-1,projectatomic/buildah,nalind/buildah,rhatdan/buildah,projectatomic/buildah,rhatdan/buildah