Add support for ARM64 Linux

[spdawson]

This depends on this PR, which adds an ARM64 Linux build of the split_tests utility.

[scruplelesswizard]

Logic here is correct (branching on $RUNNER_ARCH and picking the arm64 asset) and the upstream dependency (leonid-shevtsov/split_tests#12) is merged — but upstream's latest release (v0.5.0, published 2025-11-12) predates that merge and only ships split_tests.darwin.gz / split_tests.linux.gz — no split_tests.linux.arm64.gz asset exists in any release yet.

Merging this now means the download step 404s for arm64 runners until upstream cuts a new release containing that asset. Holding off on merging until that happens — will revisit as soon as a release with the arm64 binary ships upstream. This is also a live example of why we're moving to pin split_tests to a specific checksummed release instead of fetching /latest/download/ (tracked in a follow-up PR) — that change will make it easier to reason about exactly when it's safe to pick up arch-specific assets like this one.