Use rust-toolchain.toml before rustup on Dockerfile-{cuda,cuda-all}

[alvarobartt]

What does this PR do?

This PR adds a COPY rust-toolchain.toml rust-toolchain.toml to the Dockerfile-cuda and Dockerfile-cuda-all files, since as those are not pulling from lukemathwalker/cargo-chef:latest-rust-1.92-bookworm as the base image; rustup was installing the latest stable Rust version, which was leading to issues when building earlier versions of Text Embeddings Inference e.g.,

git clone git@github.com:huggingface/text-embeddings-inference.git
cd text-embeddings-inference/
git checkout v1.8.0
docker build --platform linux/amd64 -t text-embeddings-inference:cuda-1.8.0 -f Dockerfile-cuda-all .

Which was failing with the following exception:

  ...
  35.83 error[E0521]: borrowed data escapes outside of associated function
  35.83   --> /root/.cargo/registry/src/index.crates.io-1949cf8c6b5b557f/metrics-0.23.0/src/recorder/mod.rs:77:9
  35.83    |
  35.83 71 |       fn new(recorder: &dyn Recorder) -> Self {
  35.83    |              --------  - let's call the lifetime of this reference `'1`
  35.83    |              |
  35.83    |              `recorder` is a reference that is only valid in the associated function body
  35.83 ...
  35.83 77 | /         LOCAL_RECORDER.with(|local_recorder| {
  35.83 78 | |             local_recorder.set(Some(recorder_ptr));
  35.83 79 | |         });
  35.83    | |          ^
  35.83    | |          |
  35.83    | |__________`recorder` escapes the associated function body here
  35.83    |            argument requires that `'1` must outlive `'static`
  35.83    |
  35.83 note: raw pointer casts of trait objects cannot extend lifetimes
  35.83   --> /root/.cargo/registry/src/index.crates.io-1949cf8c6b5b557f/metrics-0.23.0/src/recorder/mod.rs:75:60
  35.83    |
  35.83 75 |         let recorder_ptr = unsafe { NonNull::new_unchecked(recorder as *const _ as *mut _) };
  35.83    |                                                            ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
  35.83    = note: this was previously accepted by the compiler but was changed recently
  35.83    = help: see <https://github.com/rust-lang/rust/issues/141402> for more information
  35.83
  35.84 For more information about this error, try `rustc --explain E0521`.
  35.84 error: could not compile `metrics` (lib) due to 1 previous error
  35.84 warning: build failed, waiting for other jobs to finish...
  63.24
  63.24 thread 'main' (7) panicked at /root/.cargo/registry/src/index.crates.io-1949cf8c6b5b557f/cargo-chef-0.1.71/src/recipe.rs:218:27:
  63.24 Exited with status code: 101
  63.24 note: run with `RUST_BACKTRACE=1` environment variable to display a backtrace
  ------
  Dockerfile-aws-cuda:52
  --------------------
    51 |
    52 | >>> RUN --mount=type=secret,id=actions_results_url,env=ACTIONS_RESULTS_URL \
    53 | >>>     --mount=type=secret,id=actions_runtime_token,env=ACTIONS_RUNTIME_TOKEN \
    54 | >>>     cargo chef cook --release --locked --recipe-path recipe.json && sccache -s;
    55 |
  --------------------
  ERROR: failed to solve: process "/bin/sh -c cargo chef cook --release --locked --recipe-path recipe.json && sccache -s;" did not
  complete successfully: exit code: 101

rustup was installing the latest stable Rust (1.86+) because no toolchain was specified, and given that Rust 1.86+ tightened lifetime checking for raw pointer casts (see rust-lang/rust#141402 and metrics-rs/metrics#564), it broken metrics-0.23.0 which was pinned in the Cargo.lock on v1.8.0.

By copying rust-toolchain.toml before any cargo invocation, rustup automatically installs and uses the exact Rust version, which fixes the immediate compile failure and prevents any future version drift.

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