Instead of rustc-1.0.0-beta-$triple.tar.gz, betas will be named
rustc-beta-$triple.tar.gz. This will give betas a stable download
URL, prevent old artifacts from accumulating in the dist server's
root directory, and not require the website to be updated every
beta.
As a tradeoff, it will be harder to download previous betas because
they will need to be located in the archives.