Add support for Linux (on 64-bit Intel/AMD)

[kring]

Copilot did the work but I checked it and it looks right to me. The CI build succeeds, but the tests fail (very quickly!) for unknown reasons. The next step is probably to try it on an actual Linux system and see what's going on.

[kring]

Oh I almost forgot about one other thing that needs to happen before this is merged. For maximum compatibility across Linux distributions, we should use a lowest common denominator build environment like Alma Linux 8 to produce our build. I think this may be as simple as adding container to the build.yml, but I haven't tried it:

    runs-on: ubuntu-latest
    container:
      image: almalinux:8