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).
Edited by Steve Lhomme