add a Rust toolchain file (!39)

fix the toolchain version of Rust with `rust-toolchain.toml`.

this is to avoid having things like `cargo clippy` behaving differently locally and in the remote CI.
> **Note**  
> see https://gitlab.isae-supaero.fr/dragoon/komodo/-/jobs/12875 which fails with Rust 1.72 but works locally with Rust 1.75