refactor: reuse nixpkgs instances

https://discourse.nixos.org/t/using-nixpkgs-legacypackages-system-vs-import/17462/8

nix/formatter/default.nix

@@ -5,11 +5,9 @@
 }: let
   inherit (nixpkgs) lib;
 in {
-  formatter = let
-    formatterOf = system: let
-      pkgs = import nixpkgs {inherit system;};
-    in
-      pkgs.alejandra;
-  in
-    lib.genAttrs systems formatterOf;
+  formatter =
+    lib.genAttrs
+    systems
+    (system:
+      nixpkgs.legacyPackages.${system}.alejandra);
 }