Split up CI jobs per OS

[pajlada]

All CI build jobs are currently in the same file.
While this reduces some code on shared steps, it makes testing artifacts more difficult and time-consuming, and makes it more difficult to add the ability to skip certain jobs for PRs.
The jobs should be split up as build-windows, build-linux, and build-macos.
We might be able to make our own 'release' local workflow that can be re-used, but if there has to be some duplicate structure, that is ok.

[pajlada]

Unfortunately, we have two issues that make this less worth doing.
A big motivation for this split would be that we'd be able to access artifacts before the build is finished, but that's not possible right now pending this feature request actions/upload-artifact#181

The alternative would be to have completely separate workflows (which was my first option), but then the release-creation becomes difficult because there's no way to fire off a workflow only once multiple workflows have succeeded pending this feature request https://github.com/orgs/community/discussions/16059

I will remove this from the milestone and mark it as blocked until one of the two features have been implemented.