nixos/dendrite: add package option

nixos/modules/services/matrix/dendrite.nix

@@ -13,6 +13,7 @@ in
 {
   options.services.dendrite = {
     enable = lib.mkEnableOption "matrix.org dendrite";
+    package = lib.mkPackageOption pkgs "dendrite" { };
     httpPort = lib.mkOption {
       type = lib.types.nullOr lib.types.port;
       default = 8008;
@@ -321,7 +322,7 @@ in
         ];
         ExecStart = lib.strings.concatStringsSep " " (
           [
-            "${pkgs.dendrite}/bin/dendrite"
+            (lib.getExe cfg.package)
             "--config /run/dendrite/dendrite.yaml"
           ]
           ++ lib.optionals (cfg.httpPort != null) [