Add tests in WSL to GitHub Actions

[sbunzel]

We know that some of our users are using Windows Subsystem for Linux and should test in that environment as well. WSL seems to be available in GitHub Actions: actions/runner-images#50 (comment)

[Vampire]

To use WSL you first have to install a distribution.
You can do so using my shiny new setup-wsl action: https://github.com/marketplace/actions/setup-wsl :-)

[klamann]

this looks very promising, thanks @Vampire