ci: resilient protoc install to fix flaky release-CDN 504s

[moonming]

Problem

arduino/setup-protoc@v3.0.0 flakes intermittently with

Error: Failed to download version v23.4: Unexpected HTTP response: 504

on protoc-23.4-linux-x86_64.zip from the GitHub release CDN (objects.githubusercontent.com). It hits whichever of the four protoc-needing jobs (lint, schema drift, rust unit + coverage, build aisix (instrumented)) loses the lottery, failing unrelated PRs. When build aisix (instrumented) is the loser, the dependent e2e + coverage >= 90% jobs are skipped, so a green-code PR shows red CI (this blocked #550 across three reruns).

repo-token is already set on all four steps — but it only authenticates the GitHub API release lookup, not the asset download, so it does not prevent the 504.

Fix

A local composite action .github/actions/setup-protoc that downloads the same pinned protoc 23.4 with hard retries (curl --retry 5 --retry-all-errors inside an outer 5× loop, 15 s backoff) and installs it under $HOME (no sudo), exposed to later steps via $GITHUB_PATH. Drop-in replacement at all four call sites (−12/+4 in ci.yml).

• Keeps protoc 23.4 — no risk of a distro protoc (e.g. the apt package on ubuntu-22.04 lacks proto3 optional) silently changing build behavior.
• DRY — one action / one version / one retry policy for all four jobs (previously four copies of the action block).
• No sudo — installs under $HOME/.local, pins PROTOC for prost/tonic build scripts.

Validation

This PR modifies the workflow, so its own CI runs every job through the new install path — if they go green (incl. build aisix (instrumented) → e2e), the fix is proven against the real flake.

Why not apt / a retry-action

• apt protobuf-compiler is the simplest "never touch the CDN" fix but changes the protoc version (version-dependent build risk) — rejected to stay zero-risk.
• A third-party retry-wrapper action adds a supply-chain dependency for what a ~14-line shell step does.

Integrity note: the asset is fetched over HTTPS from GitHub's official release CDN (TLS-authenticated), matching the integrity guarantee of the action it replaces (neither verifies a SHA-256). A pinned checksum can be added later if desired.

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Reviewing files that changed from the base of the PR and between fb1d79c and aefb179.

📒 Files selected for processing (2)

• .github/actions/setup-protoc/action.yml
• .github/workflows/ci.yml

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