ci: cache Python bindings Rust artifacts

[abnobdoss]

Which issue does this PR close?

• None. This is a CI maintenance change for the Python bindings test job.

What changes are included in this PR?

This adds a real Rust artifact cache to the Python bindings CI test job for host maturin builds on Windows and macOS.

Concretely, this PR adds two steps before PyO3/maturin-action:

• Setup Rust toolchain
• Cache Rust artifacts

Linux maturin builds may run inside the manylinux Docker container, where the host target/ cache is not expected to help. Windows and macOS build on the host, so swatinem/rust-cache can restore Cargo artifacts for the expensive maturin compile.

The cache uses a dedicated bindings key, and its save-if behavior matches the existing CI cache discipline: PRs may restore cache entries, but only main pushes create new ones.

Are these changes tested?

• Not run locally; this is a GitHub Actions workflow-only change.