Always re-generate proxy wrappers on rustup self update

[RalfJung]

Problem you are trying to solve

There are some situations where the proxy wrappers rustup generates are being overwritten -- for example, when someone works on Miri and does ./miri install. It'd be nice to have an easy way to ask rustup to re-generate these proxies.

Right now, the best advice we can give is to run rustup update, which of course as a side-effect will also update all the toolchains.

Solution you'd like

Maybe rustup self update could be changed to always re-generate the proxies, even when there is no new rustup available.

Notes

Cc rust-lang/miri#2633

[ches]

self update is often disabled for system package manager installations of rustup, so that's another situation where it being the vehicle for proxy reinstatement creates a conundrum.

Perhaps there could be a dedicated command like rustup self repair.

Refs #1305