nixos dwm: start user installed dwm if available

dwm has no configuration file. The user has to install his own version.

Changed files
+1 -1
nixos
modules
services
x11
window-managers
+1 -1
nixos/modules/services/x11/window-managers/dwm.nix
···
{ name = "dwm";
start =
''
-
${pkgs.dwm}/bin/dwm &
waitPID=$!
'';
};
···
{ name = "dwm";
start =
''
+
dwm &
waitPID=$!
'';
};