Always downloading rust toolchain

[nizasichi]

When making a new rust project with devbox. I use rustup as my tool to install all of the necessary tooling needed. However, devbox always automatically downloads the same toolchain I'd otherwise have in other projects. Is there no way for devbox to cache toolchains?

[Lagoja]

Adding @savil, who knows the rust toolchain a little better.

It may also be possible to pin specific versions of Rustc and other rust tools directly using the new version pinning features of Devbox, but we should look into caching the rustup toolchain as well.