nixos/tts: fix error messages read before text

with a config like this :

```
services.tts = {
  servers = {
    english = {
      enable = true;
      port = 5300;
      model = "tts_models/en/ljspeech/vits"
    };
  };
};
```

You the WAVs tts creates contain an error message which will be read to
you before the text you typed in will be read to you.
This patch fixes that.

nixos/modules/services/audio/tts.nix

@@ -134,6 +134,7 @@ in
           ProtectProc = "invisible";
           ProcSubset = "pid";
           RestrictAddressFamilies = [
+            "AF_UNIX"
             "AF_INET"
             "AF_INET6"
           ];