Migrate to treefmt-nix

[m15a]

error: builder for '/nix/store/wiv8iyahj0fh4bhn2la201331wsjijn1-check-format.drv' failed with exit code 1;
       last 2 log lines:
       > Passing directories or non-Nix files (such as "/nix/store/xhwpd6f9wlylhdb3a8q19g21qyn1dkkz-qjv1lq1mj7f10kdj2z769wirn4dxhipb-source/nix/") is deprecated and will be unsupported soon, please use https://treefmt.com/ instead, e.g. via https://github.com/numtide/treefmt-nix

Now that treefmt-nix is a major standard to format codes in Nix projects, let's migrate to it.