21-usersetup: check for existing /bin/zsh inside the chroot

/bin/zsh usually points to /etc/alternatives/zsh, but if
/etc/alternatives/zsh doesn't exist on the *build* system,
the test for existing /bin/zsh fails, so we need to verify
the existence of /bin/zsh *inside* the chroot.

Thanks: Krisztian VASAS <iron@ironiq.hu> for reporting the problem
Acked-by: Alexander Wirt <formorer@grml.org>