C#: Fix not assigning `runtime_initialized` when initializing with AOT.

(cherry picked from commit e868a9f577ede620aeb76a60097c470de3296233)

modules/mono/mono_gd/gd_mono.cpp

@@ -395,6 +395,7 @@ void GDMono::initialize() {
 
 		if (godot_plugins_initialize != nullptr) {
 			is_native_aot = true;
+			runtime_initialized = true;
 		} else {
 			ERR_FAIL_MSG(".NET: Failed to load hostfxr");
 		}