rust: install multiple toolchains

For the same use case in #1273 (fuzzing), allow the devcontainer to have _multiple_ rust toolchains. Fuzzing, miri, etc. require nightly rust. But other development defaults to (and can be hindered by) a move to nightly.

I admit this could be solved by two different dev containers today, but filing this issue because of the downside to that: namely, the need to maintain two separate clones, cargo caches, etc.