Support downgrading rustup from `/archive`

[rami3l]

Rustup should allow downgrading itself to an older version via the /archive path on the release server:

In fact, I have already mentioned it on Zulip:

by the way, it’s not urgent, but can we have /archive/dist on our release server redirect to /archive or something? Anyway the goal is to trick rustup into thinking the archive is an actual release server, to provide a way for arbitrary rustup downgrade/upgrade/pinning without having to adapt the code on our side

rustup could be modified to make use of the /archive path, however this modification itself would also need a new release to work.
#3844 (comment)