ci: update to actions/checkout@v6

ci is showing a lot of warnings (72) right now. apparently
actions/checkout@v4 uses Node.js 20, and all github actions are
scheduled to be force opted-in to Node.js 24 on 2026-06-02.
I don't anticipate bumping the checkout action to v6 / Node.js 24
to cause any issues (Node.js 24 drops support for ARM32 and macOS
versions <= 13.4, but this shouldn't matter because we use Docker
to test in those environments, not github runners natively) but if
it does cause issues it's probably better to find out now rather
than by surprise 3 months from now... :)