ci: improve caching of cargo tools

[kdkasad]

Uses a separate cache for Cargo-installed tools like nextest and llvm-cov so we don't have to rebuild them for each new branch/PR. Should speed up CI significantly (after this new workflow runs once).

[Copilot]

Pull request overview

This PR optimizes CI performance by implementing a dedicated caching strategy for cargo-installed tools (cargo-nextest and cargo-llvm-cov). The new approach uses a separate cache with a versioned key that persists across branches and PRs, eliminating the need to rebuild these tools for each CI run after the initial cache population.

Key changes:

• Introduces a dedicated cache for ~/.cargo/bin with manual version control in the cache key
• Disables binary caching in rust-cache to prevent conflicts with the new dedicated tool cache
• Consolidates tool installation into a single conditional step that only runs on cache misses

---

💡 Add Copilot custom instructions for smarter, more guided reviews. Learn how to get started.

[github-actions]

Test Results

46 tests   46 ✅  2s ⏱️
 1 suites   0 💤
 1 files     0 ❌

Results for commit 6d0806f.

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 86.70%. Comparing base (3fb66e2) to head (6d0806f).
⚠️ Report is 4 commits behind head on master.
✅ All tests successful. No failed tests found.

Additional details and impacted files

@@           Coverage Diff           @@
##           master      #92   +/-   ##
=======================================
  Coverage   86.70%   86.70%           
=======================================
  Files           8        8           
  Lines        2528     2528           
  Branches     2528     2528           
=======================================
  Hits         2192     2192           
  Misses        294      294           
  Partials       42       42           

☔ View full report in Codecov by Sentry.
📢 Have feedback on the report? Share it here.

🚀 New features to boost your workflow:

• 📦 JS Bundle Analysis: Save yourself from yourself by tracking and limiting bundle sizes in JS merges.