fix: replace nixfmt

[sebastianwebber]

since nixfmt package was renamed, this PR replaces the package by the new nixfmt-rfc-style.

Related to #1374

[GaetanLepage]

Thanks @sebastianwebber for your PR.
This cannot be done right now as we first need to update the flake.lock to get the revision where nixfmt has been aliased to nixfmt-classic.
The change will hence be happening in #1369.
I still don't know if we should use nixfmt-classic (what nixfmt has been aliased to, which has the same behavior) or the new nixfmt-rfc-style...

[MattSturgeon]

I still don't know if we should use nixfmt-classic (what nixfmt has been aliased to, which has the same behavior) or the new nixfmt-rfc-style...

Presumably we should offer both, but if users are using a specific formatter (classic nixfmt) then we probably shouldn't change them to a new formatter (rfc-style) without some kinda deprecation notice, warning, or opt-in...

If there's an alias then we may not need to do anything immediately?

Asside: I'm assuming nixfmt-rfc-style is functionality identical to the beta nixpkgs-fmt that was built during the RFC?

[jalil-salame]

Asside: I'm assuming nixfmt-rfc-style is functionality identical to the beta nixpkgs-fmt that was built during the RFC?

nixpkgs-fmt was built for RFC 101. RFC 101 was rejected and a rework was proposed (RFC 166). RFC 166 was accepted and is being implemented into nixfmt-rfc-style.

nixpkgs-fmt and nixfmt-rfc-style have meaningful differences.

[GaetanLepage]

I will go on and replace it with nixfmt-classic for now. Like so, we avoid breaking anything on the user side.