contrib: rust: don't download the (cross) compiler if they are missing

Only rely on the environment having the proper Rust toolchain available.

Rust contrib code will only be built if there a suitable toolchain.

Once rust in Docker images works well, we don't need to provide the rust toolchain. It's up to each developer to come up with a Rust toolchain.

We may merge this code without the Docker image updates, meaning rav1e won't be built for the time being.

Draft: on top !5779 (merged).