Cache derivations results

[SrEstegosaurio]

As the project is using nix it would be cool if it also provided a cache to avoid building it locally.
This could be done using something like cachix since they do have a free plan.

Thanks for your time and the project.

[YaLTeR]

Hey, I don't use nix myself so someone who does will need to figure it out.

As a sidenote, it would be cool to have the dev flake built and checked on CI (as allowed-fail though), but only as long as it doesn't significantly increase the build time. I suspect that will require some sort of caching.

[KiaraGrouwstra]

also see sodiboo/niri-flake#7