nixos/xmonad: Fix behavior of config opt

Prior to this change, the `config` option (which allows you define the
haskell configuration for xmonad in your configuration.nix instead of
needing something in the home directory) prevents desktop manager
resources from starting. This can be demonstrated by configuring the
following:

```
  services.xserver = {
    displayManager.defaultSession = "xfce+xmonad";
    displayManager.lightdm.enable = true;

    desktopManager.xterm.enable = false;
    desktopManager.xfce.enable = true;
    desktopManager.xfce.enableXfwm = false;
    desktopManager.xfce.noDesktop = true;

    windowManager.xmonad = {
      enable = true;
      enableContribAndExtras = true;
      extraPackages = haskellPackages: [
        haskellPackages.xmonad-contrib
        haskellPackages.xmonad-extras
        haskellPackages.xmonad
      ];
      config = ''
        import XMonad
        import XMonad.Config.Xfce
        main = xmonad xfceConfig
               { terminal = "terminator"
               , modMask = mod4Mask }
      '';
    };
  };
```

and after user log in, search for xfce processes `ps aux | grep xfce`.
You will not find xfce processes running until after the xmonad process is killed.

The bug prevents utilities included with the desktopManager,
(e.g. powerManagement, session logout, etc.)
from working as expected.

nixos/modules/services/x11/window-managers/xmonad.nix

@@ -82,12 +82,11 @@ in
     services.xserver.windowManager = {
       session = [{
         name = "xmonad";
-        start = if (cfg.config != null) then ''
-          ${xmonadBin}
-          waitPID=$!
-        '' else ''
-          systemd-cat -t xmonad ${xmonad}/bin/xmonad &
-          waitPID=$!
+        start = let
+          xmonadCommand = if (cfg.config != null) then xmonadBin else "${xmonad}/bin/xmonad";
+        in ''
+           systemd-cat -t xmonad ${xmonadCommand} &
+           waitPID=$!
         '';
       }];
     };