Revert "Fix typo in shebang"

This reverts commit 6f68bec9.

Using ash was actually correct as only this shell is available in initramfs.
It was the commit message of 27940c42 that was wrong

archiso/initcpio/script/archiso_shutdown

-#!/bin/bash
+#!/bin/ash
 
 # /oldroot depends on things inside /oldroot/run/archiso...
 mkdir /oldrun