switch from alejandra -> nixpkgs-fmt

alejandra has someone questionable choices for formatting that no one would apply to there code if they would format manually.
nixpkgs-fmt seems to produce more readable results.

flake-parts/formatting.nix

@@ -1,16 +1,15 @@
-{
-  self,
-  inputs,
-  ...
+{ self
+, inputs
+, ...
 }: {
   imports = [
     inputs.treefmt-nix.flakeModule
   ];
-  perSystem = {pkgs, ...}: {
+  perSystem = { pkgs, ... }: {
     treefmt.projectRootFile = "flake.nix";
     treefmt.flakeCheck = true;
     treefmt.flakeFormatter = true;
-    treefmt.programs.alejandra.enable = true;
+    treefmt.programs.nixpkgs-fmt.enable = true;
     treefmt.programs.shellcheck.enable = true;
   };
 }