nixos/mailman: ensure Postfix is started after Mailman

On first run, Postfix will refuse to start if it's started before
Mailman is up, because it'll try to read the map files generated
Mailman the first time it's started, and they won't exist yet.  To fix
this, make sure Postfix isn't started until after Mailman is up if
they're both activated at the same time.

nixos/modules/services/mail/mailman.nix

@@ -313,6 +313,7 @@ in {
     systemd.services = {
       mailman = {
         description = "GNU Mailman Master Process";
+        before = lib.optional cfg.enablePostfix "postfix.service";
         after = [ "network.target" ]
           ++ lib.optional cfg.enablePostfix "postfix-setup.service";
         restartTriggers = [ config.environment.etc."mailman.cfg".source ];