add a Rust toolchain file (dragoon/komodo!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

.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"
 

rust-toolchain.toml

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