[godot] Run glue generator in headless mode.

spine-godot/build/build-v4.sh

@@ -101,7 +101,7 @@ else
 	scons $target $mono_module compiledb=yes custom_modules="../spine_godot" opengl3=yes --jobs=$cpus
 	if [ $mono == "true" ]; then
 		echo "Building C# glue and assemblies."
-		"./bin/$godot_exe_host" --generate-mono-glue modules/mono/glue
+		"./bin/$godot_exe_host" --headless --generate-mono-glue modules/mono/glue
 		./modules/mono/build_scripts/build_assemblies.py --godot-output-dir ./bin --push-nupkgs-local ../godot-nuget
 	fi
 	cp compile_commands.json ../build