Add nixfmt as default formatter for nix

[TadoTheMiner]

Nix currently doesn't have a default formater so I think it should have configured nixfmt as a default formatter. This is a single line change

[t56k]

A lot of folk in Nix use Alejandra, also I think nixfmt has been deprecated

[Locorock]

A lot of folk in Nix use Alejandra, also I think nixfmt has been deprecated

last commit was last week so i doubt it. also, this NixOS/rfcs#166

[koalazub]

Looks like it's already merged into master as well: NixOS/rfcs#166

Makes sense to have it as a default and to overwrite if you wanted a different one

edit: didn't realise you referenced the 'take two' of the rfc