Multi-platform support

[ymyzk]

Switch environments based on platforms (Linux, macOS, Windows)

[ymyzk]

It seems GitHub Actions doesn't provide a platform type via environment variables: https://help.github.com/en/actions/configuring-and-managing-workflows/using-environment-variables#default-environment-variables

So, it's difficult to detect it from the plugin side. We probably need to use #20 and pass platform as an environment variable by updating the GitHub Action's configuration.

[bittner]

It seems GitHub Actions doesn't provide a platform type via environment variables:

What about the RUNNER_OS variable? Would that work?

Apart from that, what would be wrong with using Python's platform.system()?