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.

Fixes #10

alpine-make-rootfs

@@ -429,7 +429,7 @@ if [ "$SCRIPT" ]; then
 		prepare_chroot "$rootfs"
 		mount_bind "${SCRIPT%/*}" "$rootfs"/mnt
 		chroot "$rootfs" \
-			sh -c "cd /mnt && ./$script_name \"\$@\"" -- "$@" >&2 \
+			/bin/sh -c "cd /mnt && ./$script_name \"\$@\"" -- "$@" >&2 \
 			|| die 'Script failed'
 		umount_recursively "$rootfs"
 	fi