Validate nix flake in continuous integration

[gianarb]

Now that we have a nix flake for the CLI thanks to @drewr it will be nice to validate it in CI since there is automation that submits PRs to keep the various sha up to date.

[drewr]

Agree this is the next step. I'm kinda curious what's going on in #59 first because I'm not sure our test runner is working properly.