nix: format test fixtures via nix fmt

Signed-off-by: NotAShelf <raf@notashelf.dev>
Change-Id: Ia9c1e9b0a8cd9c6d834f153609baa5426a6a6964

flake.nix

@@ -4,9 +4,10 @@
   outputs = {nixpkgs, ...}: let
     systems = ["x86_64-linux" "aarch64-linux"];
     forAllSystems = nixpkgs.lib.genAttrs systems;
+    pkgsFor = system: nixpkgs.legacyPackages.${system};
   in {
     devShells = forAllSystems (system: let
-      pkgs = nixpkgs.legacyPackages.${system};
+      pkgs = pkgsFor system;
     in {
       default = pkgs.mkShell {
         name = "nixir";
@@ -34,5 +35,21 @@
         env.NIX_PLUGINABI = "0.2";
       };
     });
+
+    formatter = forAllSystems (system: let
+      pkgs = pkgsFor system;
+    in
+      pkgs.writeShellApplication {
+        name = "nix3-fmt-wrapper";
+
+        runtimeInputs = [
+          pkgs.alejandra
+          pkgs.fd
+        ];
+
+        text = ''
+          fd "$@" -t f -e nix -x alejandra -q '{}'
+        '';
+      });
   };
 }