nix/wm/xmonad: add nitrogen wallpaper manager

nix/wm/xmonad/default.nix

@@ -19,6 +19,7 @@ in
       xclip           # access x clipboard from a console
       dmenu           # menu for x window system
       flameshot       # powerful yet simple to use screenshot software
+      nitrogen        # wallpaper manager
     ];
 
     xsession = {
@@ -33,6 +34,10 @@ in
           hp.monad-logger
         ]);
       };
+
+      initExtra = ''
+        ${pkgs.nitrogen}/bin/nitrogen &
+      '';
     };
   };
 }