modules/rtirq: update usage of environment.etc

The current usage triggers the warning from
NixOS/nixpkgs@03309899:

trace: warning: In file /home/uj/nix/musnix/modules/rtirq.nix
a list is being assigned to the option config.environment.etc.
This will soon be an error as type loaOf is deprecated.
See NixOS/nixpkgs#63103 for more information.
Do
  environment.etc =
    { rtirq.conf = {...}; }
instead of
  environment.etc =
    [ { target = "rtirq.conf"; ...} ]

modules/rtirq.nix

@@ -82,11 +82,7 @@ in {
 
   config = mkIf cfg.enable {
     environment.systemPackages = [ rtirq ];
-    environment.etc =
-      [ { source = rtirqConf;
-          target = "rtirq.conf";
-        }
-      ];
+    environment.etc."rtirq.conf".source = rtirqConf;
     systemd.services.rtirq = {
       description = "IRQ thread tuning for realtime kernels";
       after = [ "multi-user.target" "sound.target" ];