Move to nixfmt

[getchoo]

see NixOS/rfcs#166. once there is a stable release, we should probably switch to it in order to conform to the wider community

[sgoudham]

If we care about the formatting of nix code. I'd like it to be automatically carried out by CI, as previously demonstrated in catppuccin/userstyles.

Carrying it out in CI removes the concern from both the maintainer and the contributor. Maintainers don't need to worry about commenting on formatting and contributors will have a smoother experience since the alternative of implementing a nixfmt lint check is frustrating if they don't have a formatter set up already.