Use rust-cache for CI Rust dependency caching

[TaKO8Ki]

This replaces the hand-written actions/cache steps with Swatinem/rust-cache@v2.

The previous workflow cached ~/.cargo/registry, ~/.cargo/git, and target separately. rust-cache handles those Rust cache paths directly, while also taking care of Rust-specific cache keys, pruning stale build artifacts, and setting CARGO_INCREMENTAL=0 for CI builds.

ref: https://github.com/swatinem/rust-cache

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[chvanam]

Independent opinion: it might not be worth taking in a third party dependency to replace 40 explicit cache lines, especially for a critical system.

[TaKO8Ki]

That’s a fair concern. As I mentioned in the PR description, this does more than replace the existing cache steps. It caches ~/.cargo and target, builds cache keys from the Rust toolchain and Cargo files, uses cargo metadata to focus on dependency artifacts, prunes stale and incremental artifacts, reuses older caches when Cargo.lock changes, works around macOS cache issues, and sets CARGO_INCREMENTAL=0.

So the value is not just fewer lines. It provides a Rust-specific caching strategy through an action already used by projects such as tokio, ruff, and uv. Since this workflow already relies on other third-party actions, I thought adding this one would be consistent with the existing configuration.