Add nix build to Github actions

[vincent-uden]

Since we're now supporting nix flakes, we want to at least try building it in the CI.

From what I understand, Nix doesn't dabble with finished binaries so the build results wont be included in the release page.

[vincent-uden]

@tukanoidd its all you, whenever you feel like you have the time :)

Also, if there's any information needed in the README for Nix folks to build the project, feel free to create a PR for that as well.