ci(docker): bump Rust from 1.74 to 1.76 (#4033)

This was accidentally missed in #3632. Jamil said it may fix
`cargo-chef`'s caching once merged.

Part of #3995

rust/Dockerfile

@@ -4,8 +4,10 @@ ARG CARGO_CHEF_VERSION="0.1.62"
 ARG RUSTUP_VERSION="1.26.0"
 ARG RUSTUP_x86_DOWNLOAD_SHA256="7aa9e2a380a9958fc1fc426a3323209b2c86181c6816640979580f62ff7d48d4"
 ARG RUSTUP_aarch64_DOWNLOAD_SHA256="b1962dfc18e1fd47d01341e6897cace67cddfabf547ef394e8883939bd6e002e"
-ARG RUST_VERSION="1.74.1"
+# Keep synced with `rust-toolchain.toml`
+ARG RUST_VERSION="1.76.0"
 
+# TODO: Document why we don't use `rust:1.76.0-alpine` here
 FROM alpine:${ALPINE_VERSION} as rust
 
 # Important!  Update this no-op ENV variable when this Dockerfile

rust/rust-toolchain.toml

@@ -1,2 +1,3 @@
 [toolchain]
+# Keep synced with `Dockerfile`
 channel = "1.76.0"