sshd.nix: Alternative fix for #19589

AFAICT, this issue only occurs when sshd is socket-activated. It turns
out that the preStart script's stdout and stderr are connected to the
socket, not just the main command's. So explicitly connect stderr to
the journal and redirect stdout to stderr.

nixos/modules/services/networking/ssh/sshd.nix

@@ -253,6 +253,10 @@ in
 
             preStart =
               ''
+                # Make sure we don't write to stdout, since in case of
+                # socket activation, it goes to the remote side (#19589).
+                exec >&2
+
                 mkdir -m 0755 -p /etc/ssh
 
                 ${flip concatMapStrings cfg.hostKeys (k: ''
@@ -270,6 +274,7 @@ in
                 KillMode = "process";
               } // (if cfg.startWhenNeeded then {
                 StandardInput = "socket";
+                StandardError = "journal";
               } else {
                 Restart = "always";
                 Type = "simple";