union: fix initialisation broken in refactor - fixes #5139

This commit broke the initialisation of the union backend

f17d7c0012 union: refactor to use fspath.SplitFs instead of fs.ParseRemote #4996

This patch fixes it.

backend/union/upstream/upstream.go

@@ -90,7 +90,7 @@ func New(ctx context.Context, remote, root string, cacheTime time.Duration) (*Fs
 		return nil, err
 	}
 	f.RootFs = rFs
-	rootString := path.Join(fsPath, filepath.ToSlash(root))
+	rootString := path.Join(remote, filepath.ToSlash(root))
 	myFs, err := cache.Get(ctx, rootString)
 	if err != nil && err != fs.ErrorIsFile {
 		return nil, err