Build linux-arm64 executable on CI

[reebalazs]

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[reebalazs]

@alex35mil hold back on this, because https://github.blog/changelog/2025-01-16-linux-arm64-hosted-runners-now-available-for-free-in-public-repositories-public-preview/ yay!

[reebalazs]

@alex35mil updated with the now-public 'ubuntu-24.04-arm` runner. This makes things simpler than in the first version of my PR that still had to use a dockerfile for the build. :)

However...

If you could build the image alex35mil/alpine-ocaml-opam-node-yarn:alpine-3.19-ocaml-4.14-node-16.20-yarn-1.22-o5gm also for platform linux/arm64/v8 then I could use this image, and we could collapse the build_linux and the build_linux_arm jobs into one job. This would be good.

Also, incidentally the validate job breaks for some reason, in the original form. So I hacked it to use the same container that I use for the arm build. This should not be needed, not sure what the proper fix is.

  E: Package 'darcs' has no installation candidate

This is a separate problem and unrelated to the arm64 build, it would fail in the master branch too.