Swap actions/checkout for useblacksmith/checkout (drop-in fork) on jobs
that run on blacksmith-*-ubuntu runners. After first-run hydration the
repo is kept as a persistent mirror on a sticky disk and each run only
fetches new commits instead of re-cloning ~280 MB from GitHub.

Left unchanged: macOS jobs, GitHub-hosted jobs (pr-size, publish_cli),
and the Windows/macOS legs of the release build matrix, since checkout
caching is backed by sticky disks (Linux runners only).

Co-Authored-By: Claude Fable 5 <noreply@anthropic.com>