action: Add setup-rust composite action

[cgwalters]

This creates a reusable composite action for setting up Rust in CI workflows. It installs the Rust toolchain, nextest, and configures caching with Swatinem/rust-cache.

This is moved out of the bootc-ubuntu-setup action because we shouldn't hardcode one thing specific to Rust there.

Assisted-by: Claude Code (Sonnet 4.5)

[gemini-code-assist]

Summary of Changes

Hello @cgwalters, I'm Gemini Code Assist¹! I'm currently reviewing this pull request and will post my feedback shortly. In the meantime, here's a summary to help you and other reviewers quickly get up to speed!

This pull request refactors CI workflows by creating a dedicated, reusable composite action for setting up the Rust development environment. This change improves the modularity and maintainability of CI configurations by isolating Rust-specific setup steps into their own action, making it easier to integrate Rust projects into various workflows without duplicating setup logic.

Highlights

• New Composite Action: A new reusable composite GitHub Action named setup-rust has been introduced. This action centralizes the setup process for Rust-based projects in CI workflows.
• Rust Setup Extraction: The Rust toolchain installation, nextest setup, and Swatinem/rust-cache configuration have been moved out of the bootc-ubuntu-setup action. This promotes better modularity and prevents hardcoding Rust-specific logic into a general setup action.
• Standardized Rust CI: The new setup-rust action provides a consistent way to prepare a CI environment for Rust projects, including toolchain installation, test runner setup, and efficient caching strategies.

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[gemini-code-assist]

Code Review

This pull request introduces a new composite action setup-rust to centralize the setup of the Rust toolchain, nextest, and caching. This is a good refactoring that improves reusability and separates concerns from the bootc-ubuntu-setup action. My review includes a suggestion to improve the robustness of the new action by pinning one of the used actions to a major version, which is a best practice for GitHub Actions to ensure workflow stability. The rest of the implementation looks solid.