Set up CI using GitHub actions and Nix

[freeman42x]

Use whichever CI you decided on after doing research in #143

Tasks:

• What you need to do is to do a nix-build in the project root. This requires having nix installed, cachix set up, etc.
• Have a look at how jappiejappie did the CI here for TraviCI, for GitHub Actions there are probably a lot of similarities: https://github.com/jappeace/cut-the-crap/blob/master/.travis.yml#L7 Example CI output: https://travis-ci.org/github/jappeace/cut-the-crap

If using TravisCI requires any account create one for yourself and use it or ask me to create one for the organization.