Call shell by absolute path in chroot

When using the --script-chroot option, chroot is called with just sh
(relative path), relying on the host environment's $PATH to find the
executable. At least on Arch Linux, this does not work, as Arch Linux
by default does not have /bin in the PATH (/bin is just a symlink to
/usr/bin).

Since the chroot system was just built and it is know to contain
/bin/sh, I think it makes sense to just call it explicitly here.

Related to https://github.com/alpinelinux/alpine-make-rootfs/issues/10



Co-Authored-By: Conrad Hoffmann <[email protected]>