Use alejandra for formatting Nix files

flake.nix

@@ -1,5 +1,4 @@
 # Do not edit this file. It is automatically generated by https://www.oliverdavies.uk/build-configs.
-
 {
   inputs = {
     devshell.url = "github:numtide/devshell";
@@ -12,7 +11,14 @@
 
       systems = ["x86_64-linux"];
 
-      perSystem = { config, self', inputs', pkgs, system, ... }: {
+      perSystem = {
+        config,
+        self',
+        inputs',
+        pkgs,
+        system,
+        ...
+      }: {
         devshells.default = {
           packages = with pkgs; [
             "nodePackages.pnpm"
@@ -21,6 +27,8 @@
             "php81Packages.composer"
           ];
         };
+
+        formatter = pkgs.alejandra;
       };
     };
 }