nixos/roon-server: persist installation ID to avoid re-login

Set environment variable ROON_ID_DIR to the same value as ROON_DATA_DIR
so that it knows it's the same installation after a restart.
Otherwise, each time the server process restarts, the user will need
to log in, and configure any server-local audio devices, again.

This environment variable was found in Roon's "easy installer" script:
https://help.roonlabs.com/portal/en/kb/articles/linux-install#The_Easy_Installer_Recommended

nixos/modules/services/audio/roon-server.nix

@@ -40,6 +40,7 @@ in {
       wantedBy = [ "multi-user.target" ];
 
       environment.ROON_DATAROOT = "/var/lib/${name}";
+      environment.ROON_ID_DIR = "/var/lib/${name}";
 
       serviceConfig = {
         ExecStart = "${pkgs.roon-server}/bin/RoonServer";