nix: configure and apply a formatter

This sets the formatter to alejandra. I was trying to stay as close as
the original as I could; otherwise we could set it to `nixfmt` which
would keep list spacing more similar, but would introduce additional
indentation in a few places.

flake.nix

@@ -6,20 +6,27 @@
     flake-utils.url = "github:numtide/flake-utils";
   };
 
-  outputs = { self, nixpkgs, flake-utils }:
-    flake-utils.lib.eachDefaultSystem (system:
-      let
+  outputs = {
+    self,
+    nixpkgs,
+    flake-utils,
+  }:
+    flake-utils.lib.eachDefaultSystem (
+      system: let
         pkgs = import nixpkgs {inherit system;};
 
         # Python with the packages needed by the offline converter tools
-        pythonEnv = pkgs.python3.withPackages (ps: with ps; [
+        pythonEnv = pkgs.python3.withPackages (
+          ps:
+            with ps; [
               torch
               safetensors
               huggingface-hub
               numpy
               tokenizers
               datasets
-        ]);
+            ]
+        );
 
         colibri = pkgs.stdenv.mkDerivation {
           pname = "colibri";
@@ -107,6 +114,8 @@
           };
         };
 
+        formatter = (import nixpkgs {inherit system;}).alejandra;
+
         devShells.default = pkgs.mkShell {
           inputsFrom = [colibri];