nix: Switch formatter from nixpkgs-fmt to nixfmt

nixpkgs-fmt was archived[1] in favour of nixfmt[2], which implements the
Nix RFC 166[3].

See NixOS/nixfmt#153 for further details about the progress of the
implementation.

[1]: https://x.com/zimbatm/status/1816148339131343058
[2]: https://github.com/NixOS/nixfmt?tab=readme-ov-file#readme
[3]: https://github.com/nix-rfc-101/rfcs/blob/aaf1613f/rfcs/0166-nix-formatting.md

flake.nix

@@ -15,30 +15,44 @@
 
   inputs.flake-schemas.url = "https://flakehub.com/f/DeterminateSystems/flake-schemas/0.1.*.tar.gz";
 
-  outputs = { self, nixpkgs, ruby-nix, bundix, flake-schemas }:
+  outputs =
+    {
+      self,
+      nixpkgs,
+      ruby-nix,
+      bundix,
+      flake-schemas,
+    }:
     let
       allSystems = [
         "x86_64-linux"
         "aarch64-darwin"
       ];
 
-      forAllSystems = f: nixpkgs.lib.genAttrs allSystems (system: f {
-        # Current 'ruby' package version: 3.3.4
-        # Should match https://github.com/actions/jekyll-build-pages/blob/v1.0.13/Dockerfile
-        pkgs = import nixpkgs { inherit system; };
-      });
+      forAllSystems =
+        f:
+        nixpkgs.lib.genAttrs allSystems (
+          system:
+          f {
+            # Current 'ruby' package version: 3.3.4
+            # Should match https://github.com/actions/jekyll-build-pages/blob/v1.0.13/Dockerfile
+            pkgs = import nixpkgs { inherit system; };
+          }
+        );
     in
     {
       inherit (flake-schemas) schemas;
 
-      formatter = forAllSystems ({ pkgs }: pkgs.nixpkgs-fmt);
+      formatter = forAllSystems ({ pkgs }: pkgs.nixfmt-rfc-style);
 
-      devShells = forAllSystems ({ pkgs }:
+      devShells = forAllSystems (
+        { pkgs }:
         let
-          ruby-env = (ruby-nix.lib pkgs {
-            name = "github-pages";
-            gemset = ./gemset.nix;
-          }).env;
+          ruby-env =
+            (ruby-nix.lib pkgs {
+              name = "github-pages";
+              gemset = ./gemset.nix;
+            }).env;
 
           bundix-cli = bundix.packages.${pkgs.system}.default;