Building cbindgen takes a few minutes on Windows, so avoid that by using
the version included in the GitHub Actions runner.
The version include in the runners isn't the same as what the build
expects, but hopefully the differences aren't significant.
AppVeyor is breaking my builds because I can't create any more artifacts
because I've hit the limit, but there's no way to have it not store
artifacts or to manually delete artifacts...
This also updates Boost from 1.67.0 to 1.69.0 due to differences between
the AppVeyor and GitHub Actions build environments.