ci: fix macOS rustup cache corruption in install-rust action

[GuillaumeLagrange]

Switch the install-rust composite action back to rustup toolchain install and disable cache-bin on the rust-cache step for macOS.

macOS lint jobs were failing with:

error: unexpected argument 'fmt' found
Usage: rustup-init[EXE] [OPTIONS]

Two compounding causes:

• rustup show was used to materialize the toolchain pinned in rust-toolchain.toml. Per the rustup 1.28 release notes, rustup show no longer auto-installs the active toolchain — the documented replacement is rustup toolchain install (with no arguments). Behavior was partially restored in 1.28.1 behind RUSTUP_AUTO_INSTALL, so relying on rustup show is fragile across runner image updates.
• Swatinem/rust-cache caches ~/.cargo/bin by default. On macOS, the cache had been saved on a runner with a different rustup version; on restore it overwrote the freshly-installed cargo proxy with a stale rustup-init binary, which is why cargo fmt/cargo clippy reported Usage: rustup-init[EXE].

This matches the pattern tokio uses (tokio-rs/tokio CI): install the toolchain first, then run rust-cache with cache-bin: false on macOS only.

Tested with 10 runs of just the macos lint

[codspeed-hq]

Merging this PR will not alter performance

⚠️ Unknown Walltime execution environment detected

Using the Walltime instrument on standard Hosted Runners will lead to inconsistent data.

For the most accurate results, we recommend using CodSpeed Macro Runners: bare-metal machines fine-tuned for performance measurement consistency.

✅ 7 untouched benchmarks

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_{Comparing fix-ci-rustup-macos (0d87670) with main (149a618)}