ci: skip root tty login

We use the `autologin` mkosi option (see
mkosi.default.d/10-systemd.conf), so the pexpect root login throws
a (harmless) error:

```
Arch Linux (built from systemd tree)
Kernel 5.4.0-1047-azure on an x86_64 (console)

image login: root (automatic login)

root
root
[root@image ~]# systemctl poweroff
root
-bash: root: command not found
[root@image ~]# systemctl poweroff
```