LibWeb: Remove extraneous trailing '/' from service worker script scope

The spec language specifies 'Set maxScopeString to "/", followed by the
strings in XXXX’s path (including empty strings), separated from each
other by "/"': That is, adjacent components are separated by a '/', but
the last component does not get a trailing '/'.

This resulted in the generated scope string ending with '//' in some
cases, incorrectly tripping the 'Service-Worker-Allowed' security check

Libraries/LibWeb/ServiceWorker/Job.cpp

@@ -289,10 +289,7 @@ static void update(JS::VM& vm, GC::Ref<Job> job)
             auto join_paths_with_slash = [](URL::URL const& url) -> ByteString {
                 StringBuilder builder;
                 builder.append('/');
-                for (auto const& component : url.paths()) {
-                    builder.append(component);
-                    builder.append('/');
-                }
+                builder.join('/', url.paths());
                 return builder.to_byte_string();
             };