rspamd: make sure redis is started over TCP socket

mail-server/rspamd.nix

@@ -99,6 +99,12 @@ in
     };
 
     services.redis.enable = true;
+    assertions = [
+      { 
+        assertion = isNull config.services.redis.unixSocket;
+        message = "nixos-mailserver doesn't support redis over UNIX socket";
+      }
+    ];
 
     systemd.services.rspamd = {
       requires = [ "redis.service" ] ++ (lib.optional cfg.virusScanning "clamav-daemon.service");