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

.gitlab-ci.yml

@@ -5,7 +5,7 @@ stages:
   - test
 
 variables:
-  RUSTUP_CHANNEL: "1.72.1"
+  RUSTUP_CHANNEL: "1.75"
   NUSHELL_ARCH: "x86_64-unknown-linux-musl"
   NUSHELL_VERSION: "0.89.0"