[ci] Download build artifacts in the background during job setup (#37350)

Every downstream job in `runtime_build_and_test.yml` restored the 50
`_build_*` artifacts with `actions/download-artifact` only after
setup-node, the node_modules cache restore, and any installs had
completed, even though the download is independent of all of them.

This change marks the download as a [background
step](https://docs.github.com/en/actions/reference/workflows-and-actions/workflow-syntax#jobsjob_idstepsbackground)
started immediately after checkout, and adds an explicit `wait:
download_build` before the first step that reads `build/`.

The download starts after checkout because `actions/checkout` runs `git
clean`, which would wipe a previously downloaded `build/` directory. The
`sizebot` job is unchanged because its base-build download also writes
`./build` and would collide with a concurrent artifact restore.

This only shaves of a few seconds from wall time. It's more about
establishing precedent.

Co-authored-by: Claude Code (kimi-k3[1m]) <noreply@anthropic.com>