`multirust update nightly-$date` should not redownload if that nightly is already installed

If a nightly (or any specific version) is already installed, it shouldn't need to be redownloaded each time; I believe there's a hash that could be downloaded first and compared?

Start by tracing the `multirust update nightly-$date` entry point and the existing logic for checking installed specific versions. Verify how downloads are selected and how an installed nightly is identified. Done means repeating the update for an installed nightly avoids downloading it while still handling a missing nightly correctly.

Written by the indexing model from the issue text.