rust toolchain is spread accross multiple files
!39 (merged) introduces a new rust-toolchain.toml file that fixes the Rust toolchain to some version.
but there is also $RUSTUP_CHANNEL that is defined in .gitlab-ci.yml which might conflict with the value in rust-toolchain.toml...
we might want to make sure there is a single entry point (and point of failure) for the Rust toolchain.
