nixos/cage: add package option

Adds a package option so users can inject their own package. Previously
the only way to change the package was to use an overlay and change the
cage package everywhere.

nixos/modules/services/wayland/cage.nix

@@ -41,6 +41,8 @@ in {
     '';
   };
 
+  options.services.cage.package = mkPackageOption pkgs "cage" { };
+
   config = mkIf cfg.enable {
 
     # The service is partially based off of the one provided in the
@@ -64,7 +66,7 @@ in {
       unitConfig.ConditionPathExists = "/dev/tty1";
       serviceConfig = {
         ExecStart = ''
-          ${pkgs.cage}/bin/cage \
+          ${cfg.package}/bin/cage \
             ${escapeShellArgs cfg.extraArguments} \
             -- ${cfg.program}
         '';