mkosi: use systemd.firstboot=no to turn of interactivity at boot

Now that creds are processed even if systemd.firstboot=no is set, we can
use it to disable the root pw prompt *and* the new homectl prompt at the
same time, without breaking the creds stuff.

mkosi.conf

@@ -18,8 +18,6 @@ Environment=ASAN_OPTIONS=verify_asan_link_order=false
 @Incremental=yes
 @QemuMem=2G
 @RuntimeSize=8G
-# Make sure we don't trigger systemd-firstboot prompting for the root password.
-Credentials=passwd.plaintext-password.root=
 KernelCommandLineExtra=systemd.crash_shell
                        systemd.log_level=debug
                        systemd.log_ratelimit_kmsg=0
@@ -37,3 +35,4 @@ KernelCommandLineExtra=systemd.crash_shell
                        selinux=0
                        enforcing=0
                        systemd.early_core_pattern=/core
+                       systemd.firstboot=no