* Show default outputs
Following https://github.com/NixOS/nix/pull/6242, we now have access to
default outputs so we can single them out in the frontend.
We can also drop `--out-path`, which speeds up import quite a lot and
doesn't require increasing swap space or allowing broken/unfree
packages anymore.
* Use nixUnstable in CI
* Bump VERSION
* Add github action to check flake groups
* Run on pull_requests and filter ~files~ *paths*
* Use composite action to reuse setup steps
* Use common setuo for frontend action
* Fix path to action
* Remove checkout action from common setup
Checkout needed to fetch action
* Provide secrets to action
Co-authored-by: Rok Garbas <rok@garbas.si>
