main/mkinitfs: depend on busybox-binsh

An explicit dependency on busybox-binsh can solve issues when e.g. dash-binsh was chosen by apk to fulfil the dependency on /bin/sh. However, dash is not fully compatible with mkinitfs and broken initramfs could result from such instances.

Note that while busybox-binsh does have a higher priority that dash-binsh, apk may still choose dash-binsh to fulfil a dependency to /bin/sh if a package is installed that conflicts with busybox-binsh.