workflows/tests: install actionlint.

[MikeMcQuaid]

This is otherwise installed by brew style.

---

• Have you followed the guidelines in our Contributing document?
• Have you checked to ensure there aren't other open Pull Requests for the same change?
• Have you added an explanation of what your changes do and why you'd like us to include them? Performance claims (e.g. "this is faster") must include Hyperfine benchmarks.
• Have you written new tests (excluding integration tests) for your changes? Here's an example.
• Have you successfully run brew lgtm (style, typechecking and tests) with your changes locally?

---

• AI was used to generate or assist with generating this PR. Please specify below how you used AI to help you, and what steps you have taken to manually verify the changes. Non-maintainers may only have one AI-assisted/generated PR open at a time.

Done by ✋🏻

---

[Copilot]

Pull request overview

This PR ensures the CI syntax job preinstalls actionlint alongside the existing shell/style tools so brew style does not need to install it during the run.

Changes:

• Adds actionlint to the Homebrew prefix cache install list for the syntax job.
• Updates the corresponding step name to reflect the expanded tool set.

---

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