nixos/spotifyd: add package option

Spotify service changes frequently break spotifyd. Using newer releases
from upstream requires overlaying the module too, even though it rarely
changes package. This change should allow using newer spotifyd builds (e.g.
from nixos-unstable) without having to overlay the module in the current
(e.g. nixos stable) system.

nixos/modules/services/audio/spotifyd.nix

@@ -23,6 +23,8 @@ in
     services.spotifyd = {
       enable = lib.mkEnableOption "spotifyd, a Spotify playing daemon";
 
+      package = lib.mkPackageOption pkgs "spotifyd" { };
+
       config = lib.mkOption {
         default = "";
         type = lib.types.lines;
@@ -43,6 +45,7 @@ in
           <https://docs.spotifyd.rs/configuration/index.html#config-file>.
         '';
       };
+
     };
   };
 
@@ -64,7 +67,7 @@ in
       description = "spotifyd, a Spotify playing daemon";
       environment.SHELL = "/bin/sh";
       serviceConfig = {
-        ExecStart = "${pkgs.spotifyd}/bin/spotifyd --no-daemon --cache-path /var/cache/spotifyd --config-path ${spotifydConf}";
+        ExecStart = "${cfg.package}/bin/spotifyd --no-daemon --cache-path /var/cache/spotifyd --config-path ${spotifydConf}";
         Restart = "always";
         RestartSec = 12;
         DynamicUser = true;