Migrate to nixfmt

[rohaquinlop]

nixfmt will form the basis for the initial official standard Nix formatter, as established by RFC 166. We should migrate from Alejandra to nixfmt

[blotero]

nixfmt can only fail when the check flag is used, CI systems which rely on a job with the /formatNix builtin would not fail under non compliant nix code. Is this intentional?

https://github.com/NixOS/nixfmt/blob/master/main/Main.hs#L59