audio: try to work around nssmdns4 only available on nixos-unstable

Closes #5

modules/audio.nix

@@ -1,4 +1,4 @@
-{ config, lib, pkgs, ... }:
+{ config, lib, options, pkgs, ... }:
 
 let
   cfg = config.c3d2.audioStreaming;
@@ -59,9 +59,11 @@ in
     security.rtkit.enable = true;
 
     services = {
-      avahi = {
+      avahi = let
+        nssmdns = if lib.hasAttr "nssmdns4" options.services.avahi then "nssmdns4" else "nssmdns";
+      in {
         enable = true;
-        nssmdns4 = true;
+        "${nssmdns}" = true;
       };
 
       pipewire.pulse.enable = true;