Explicitly copy keyscript

We need to use a path that exists in both initramfs and rootfs since cryptsetup-initramfs will otherwise not add the crypttab entry for "keyscript=...,initramfs" crypttab entries.

This allows crypttab enries like

crypt_root UUID=9181cc29-72f2-4c1f-873a-2cfda7a277e2 none luks,discard,keyscript=/usr/share/initramfs-tools/scripts/osk-sdl-keyscript,initramfs

to work.