Split Github Action CI for each target platform

[rtk0c]

Currently Linux, Windows, and macOS builds are run in sequence in the same action. This speeds up CI speed a lot (70% of our time is spent in initializing npm and go) but makes a platform-specific error in, for example Linux, to shortcircuit the action immediately.

It is up to debate which way it should be. Since there is no CGO or other platform-specific code involved currently, the benefit here is quite low. It may have in the future.