Add customize_airootfs.sh to remove root password on live system

Without this, pacstrap locks root's password by default and the live
ISO is unbootable/unloggable-into.

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

iso/profile/airootfs/root/customize_airootfs.sh

@@ -0,0 +1,5 @@
+#!/usr/bin/env bash
+set -e -u
+
+# Allow passwordless root login on the live system
+passwd -d root