Mention versioned rustup-init binary URLs

https://forge.rust-lang.org/infra/other-installation-methods.html#other-ways-to-install-rustup should probably mention that it's possible to download specific versions of `rustup-init` binaries, via the versioned download links mentioned in the similar docs at https://rust-lang.github.io/rustup/installation/other.html#manual-installation. I noticed this because I'm trying to set up a reproducible build for a rust toolchain at my company. https://www.rust-lang.org/tools/install links to https://forge.rust-lang.org/infra/other-installation-methods.html, so this seems to be the most authoritative version of the install instructions. And these instructions currently suggest only the unversioned `rustup-init` URLS, which are presumably not fit for a reproducible build because they're going to get updated over time to point to the latest builds.