rqbit: expose the webui as passthru

this can be useful for ex. for `nix develop .#rqbit.webui` or perhaps if
one needs to server the static pages from another http server

pkgs/by-name/rq/rqbit/package.nix

@@ -68,6 +68,8 @@ rustPlatform.buildRustPackage {
 
   doCheck = false;
 
+  passthru.webui = rqbit-webui;
+
   meta = with lib; {
     description = "Bittorrent client in Rust";
     homepage = "https://github.com/ikatson/rqbit";