shell: fix overlay and update

shell.nix

@@ -3,7 +3,7 @@
   pkgs ?
     import sources.nixpkgs {
       overlays = map import [
-        ./nix/overlays
+        ./nix/overlays/disko.nix
         ./nix/overlays/packages.nix
       ];
     },
@@ -17,6 +17,16 @@ pkgs.mkShell {
     just
     jq
 
+    #
+    # XMonad
+    #
+    (haskellPackages.ghcWithPackages (self: [
+      self.xmonad-contrib
+      self.xmonad-extras
+    ]))
+    haskell-language-server
+    cabal-install
+
     #
     # Installer testing
     #