nixos/dovecot: set group in config

The dovecot bump to 2.3.1 caused the dovecot service to fail to start
because it would try to chgrp sockets to dovecot whereas our default
dovecot group is called dovecot2.

nixos/modules/services/mail/dovecot.nix

@@ -30,6 +30,7 @@ let
 
     ''
       default_internal_user = ${cfg.user}
+      default_internal_group = ${cfg.group}
       ${optionalString (cfg.mailUser != null) "mail_uid = ${cfg.mailUser}"}
       ${optionalString (cfg.mailGroup != null) "mail_gid = ${cfg.mailGroup}"}