nixos/vikunja: install 'vikunja' CLI tool

See https://vikunja.io/docs/cli/

nixos/modules/services/web-apps/vikunja.nix

···
     };
 
     environment.etc."vikunja/config.yaml".source = configFile;
+
+    environment.systemPackages = [
+      cfg.package-api # for admin `vikunja` CLI
+    ];
   };
 }