tuftool: `download_root()` function should handle `file:///` URLs

[Zac]

Currently the download_root::download_root() function uses reqwest to pull down the root.json. reqwest doesn't handle file:/// URLs and so unfortunately this means the function can't be used for local repositories which makes unit tests a bit... tough.

The function should probably (suggested by @tjkirch in #404 ) add a branch, where if url.scheme() is file, we just std::fs::copy it instead.