shell: use nix-shell args to call

This makes the different shell options easier to compose with

shell.nix

@@ -7,27 +7,24 @@
         ./nix/packages/overlay.nix
       ];
     },
-}: rec {
+  withXMonad ? false,
-  default = pkgs.mkShell {
+}: let
-    packages = with pkgs; [
+  inherit (pkgs) lib;
-      just
+in
-      jq
+  pkgs.mkShell {
-      npins
+    packages = with pkgs;
-      disko
+      [
-    ];
+        just
-  };
+        jq
-
+        npins
-  withXMonad = pkgs.mkShell {
+        disko
-    inputsFrom = [
+      ]
-      default
+      ++ lib.optionals withXMonad [
-    ];
+        (haskellPackages.ghcWithPackages (self: [
-    packages = with pkgs; [
+          self.xmonad-contrib
-      (haskellPackages.ghcWithPackages (self: [
+          self.xmonad-extras
-        self.xmonad-contrib
+        ]))
-        self.xmonad-extras
+        haskell-language-server
-      ]))
+        cabal-install
-      haskell-language-server
+      ];
-      cabal-install
+  }
-    ];
-  };
-}