shell: use nix-shell args to call

This makes the different shell options easier to compose with

shell.nix

@@ -7,21 +7,19 @@
         ./nix/packages/overlay.nix
       ];
     },
-}: rec {
-  default = pkgs.mkShell {
-    packages = with pkgs; [
+  withXMonad ? false,
+}: let
+  inherit (pkgs) lib;
+in
+  pkgs.mkShell {
+    packages = with pkgs;
+      [
         just
         jq
         npins
         disko
-    ];
-  };
-
-  withXMonad = pkgs.mkShell {
-    inputsFrom = [
-      default
-    ];
-    packages = with pkgs; [
+      ]
+      ++ lib.optionals withXMonad [
         (haskellPackages.ghcWithPackages (self: [
           self.xmonad-contrib
           self.xmonad-extras
@@ -29,5 +27,4 @@
         haskell-language-server
         cabal-install
       ];
-  };
-}
+  }