ci(pr-review-companion): run independent steps in parallel

[caugner]

Description

Update the PR review companion workflow to run independent, network-bound steps in parallel using the GitHub Actions parallel step group:

• the artifact download and the mdn/content checkout,
• the status update, GCP authentication, and Node setup.

Motivation

Speed up CI by overlapping independent download, checkout, and setup steps instead of running them serially.

Additional details

Uses the parallel step keywords introduced in GitHub Actions on 2026-06-25: https://github.blog/changelog/2026-06-25-actions-steps-can-now-be-run-in-parallel/

parallel is shorthand for background: true on each step followed by an implicit wait: https://docs.github.com/en/actions/reference/workflows-and-actions/workflow-syntax#jobsjob_idstepsparallel

Measured over the last 25 runs of the review job, this saves about 4s of a ~48s job (8%), consistently 7-9% per run. Group 1 collapses Download artifact (1-3s) into Checkout (mdn/content) (4-11s); group 2 collapses Mark status as pending (1-4s) and Authenticate with GCP (0-1s) into Setup (mdn/content) (4-5s).

One behavior change: Checkout (mdn/content) is no longer gated on HAS_ARTIFACT, because it now precedes the Check for artifacts step. It runs concurrently with the artifact download, so there is no wall-clock cost.

The other workflows in this repository were checked and left unchanged. interfacedata-updater.yml would save ~8s but only runs weekly, and pr-reviewdog.yml saves a median of 0s (its artifact download is under a second, and Setup reviewdog executed in only 2 of 25 sampled runs). The rest, including test.yml, markdown-lint.yml, the pr-check_*.yml set, spelling-check-bot.yml, and auto-cleanup-bot.yml, are strict checkout -> setup-node chains: node-version-file: .nvmrc and cache: npm both need the checkout to have finished, so there is nothing independent to overlap.

Note that this workflow triggers on workflow_run only, so it does not run on this PR.

Related issues and pull requests

Part of mdn/fred#1862.

Follows mdn/rari#775.