Cache the Rust toolchain in CI (#15660)
We're spending a full 1.5m installing the Rust toolchain on Windows,
e.g.,
https://github.com/astral-sh/ruff/actions/runs/12893749773/job/35950838258
In contrast, in uv this is instant (e.g.
https://github.com/astral-sh/uv/actions/runs/12897572530/job/35962989190)
because we are caching the toolchain.
This shifts the rust-cache action earlier in all the CI jobs.
