ci: replace arduino/setup-protoc@v3 with direct install before GitHub Actions Node.js 24 cutover (2026-06-02)

[mosuka]

Background

GitHub Actions is switching its default Node.js runtime to 24 on 2026-06-02. arduino/setup-protoc@v3 pins Node.js 20 internally; after the cutover it will start failing on every workflow that uses it. The latest release of the action remains v3.0.0 (2024-01-31) — no v4 with Node.js 24 support exists yet, and the upstream tracking ticket (arduino/setup-protoc#113) has not moved.

We use this action in 10 workflow steps:

• .github/workflows/release.yml — 2 occurrences
• .github/workflows/regression.yml — 4 occurrences
• .github/workflows/periodic.yml — 4 occurrences

If we wait and v4 does not ship in time, releases and PR validation both break on or shortly after 2026-06-02.

Goal

Remove the dependency on arduino/setup-protoc@v3 by installing the protoc binary directly in each workflow step that needs it, before the Node.js 24 cutover.

Proposed approach

Add a small reusable "install protoc" step (composite action under .github/actions/install-protoc/ or an inline shell snippet) that selects the right installer per OS:

• Linux runners (ubuntu-*, *-arm): apt-get install -y protobuf-compiler (matches the protoc version Debian/Ubuntu ships) or download an official protoc-*-linux-x86_64.zip / protoc-*-linux-aarch_64.zip release asset if a more precise version pin is desired.
• macOS runners (macos-*): brew install protobuf.
• Windows runners (windows-*): choco install -y protoc, or download protoc-*-win64.zip from the official release assets.

The official release-asset path is preferred over package managers because it gives us a deterministic version pin (matches how arduino/setup-protoc itself works). Pin the version to the same one currently requested via the version: input to arduino/setup-protoc (audit per-step before swap).

Replace all 10 call sites in one PR so the migration is atomic and the diff is easy to review.

Acceptance criteria

• No remaining references to arduino/setup-protoc under .github/workflows/.
• protoc --version step in CI reports the same major version we used before the swap.
• release.yml, regression.yml, periodic.yml all execute through to the same outcome they had on the last green run (no new failures attributable to the protoc swap).
• Works on Linux x86_64, Linux aarch64, macOS, and Windows runners — the matrix entries we already have.
• If a composite action is used, it lives under .github/actions/install-protoc/ with its own action.yml and is documented at the top of the file.

Out of scope

• Switching to a different protobuf compiler entirely (buf, prost-build only, etc.).
• Touching the Rust-side tonic-build / prost-build integration.

References

• GitHub Actions Node.js 24 cutover: https://github.blog/changelog/2025-09-29-github-actions-default-runtime-for-actions-moving-to-node24/
• Upstream tracking ticket: Upgrade to Node 24 arduino/setup-protoc#113
• protoc release assets: https://github.com/protocolbuffers/protobuf/releases

Deadline

2026-06-02 — the day GitHub Actions changes its default Node.js runtime. Land before this.