ci: parallelize independent setup steps in build and test jobs - #52721

Requested by Samuel Attard · Slack thread

Description of Change

Before: the test jobs run five independent setup steps (installing dependencies, cloning depot_tools, downloading the generated and src artifacts, and provisioning the Linux CUPS virtual printer) strictly one after another, and macOS builds wait for the disk-freeing step to finish before doing anything else — including restoring caches that don't need the freed space yet.

After: those steps overlap using GitHub Actions' new parallel/background steps, saving roughly 4 minutes of wall time per full run with no behavior change — every step still runs to completion before anything that depends on it.

How: the five independent setup steps in the test segment are grouped under a parallel: block (they write disjoint paths and none reads another's output; env- and PATH-mutating steps stay serial and are only consumed after the group's implicit wait). In the build segment, the macOS disk-freeing step is backgrounded with background: true and a wait-all fence is placed just before the src-cache restore — the step that actually needs the freed disk (the fence is a no-op on Windows/Linux). The publish segment is regenerated from the build segment via the existing sync script, so it picks up the same change.

Deliberately left serial: the Add Depot Tools to PATH and other GITHUB_ENV/GITHUB_PATH-mutating steps, and the macOS AZCopy install, which must not overlap the brew invocations inside the disk-freeing step.

Checklist

• I have built and tested this change
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• I have reviewed and verified the changes
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Release Notes

Notes: none

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