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
```

.github/workflows/test_mkosi_boot.py

@@ -8,9 +8,6 @@ import sys
 def run() -> None:
     p = pexpect.spawnu(" ".join(sys.argv[1:]), logfile=sys.stdout, timeout=300)
 
-    p.expect("login:")
-    p.sendline("root")
-
     p.expect("#")
     p.sendline("systemctl poweroff")