Standalone builds for Linux aarch64 (aka arm64)

[tsibley]

GitHub released public runners.¹ This will ultimately let us use standalone installation archives in our linux/arm64 Docker images.²

Changes to the standalone installer to follow separately³, as those need to be held for merge until after the first release with these builds available.

¹ https://github.blog/changelog/2025-01-16-linux-arm64-hosted-runners-now-available-for-free-in-public-repositories-public-preview/ ² https://github.com/nextstrain/docker-base/pull/273 ³ https://github.com/nextstrain/cli/pull/490

Checklist

• [ ] Checks pass
• [ ] Update changelog