Add live-environment config so the ISO boots straight to the session

The fixed initramfs boots into userspace, but systemd-firstboot
(ConditionFirstBoot=yes, --prompt-locale --prompt-keymap-auto
--prompt-timezone --prompt-root-password) then blocked the console
waiting for interactive input, and root was locked (no /etc/shadow),
so the live medium never reached the autologin getty + Hyprland.

Ship the same base files releng uses to satisfy firstboot and unlock
root for autologin:
- etc/locale.conf  (LANG=C.UTF-8)        -> no locale prompt
- etc/localtime    (-> UTC)              -> no timezone prompt
- etc/vconsole.conf (KEYMAP=us)          -> no keymap prompt
- etc/hostname     (bos)
- etc/shadow       (root unlocked, empty pw, perms 0400 via profiledef)
- etc/passwd       (root shell = bash; system users are appended by the
                    systemd-sysusers pacman hook during pacstrap)

The overlay is applied before pacstrap (mkarchiso _make_custom_airootfs
precedes _make_packages) and these are pacman backup files, so the
static passwd/shadow act as the base and package scriptlets add the
rest — no clobbering of polkitd/pipewire/etc. users.

iso/airootfs/etc/passwd

@@ -0,0 +1 @@
+root:x:0:0:root:/root:/usr/bin/bash