add a Rust toolchain file

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

rust-toolchain.toml

+[toolchain]
+profile = "default"
+channel = "1.75"