### Godot version

4.0 Stable

### System information

Ubuntu 22.04

### Issue description

Setting fog_affect_sky = 1 do not have the desired effect when using a Color sky mode. It works properly if we use a procedural sky or the forward+ renderer.

![Screenshot from 2023-03-03 16-50-53](https://user-images.githubusercontent.com/8985409/222870514-0195c4cb-bbca-4e9b-bb6a-6e2a568d2f7b.png)


### Steps to reproduce

Add a sky with a plain color mode like the Clear Color default option. Enable fog and set fog_affect_sky value to 1. Sky is still not affected by fog. 

### Minimal reproduction project
[mobile_fog_sky_bug.zip](https://github.com/godotengine/godot/files/10887265/mobile_fog_sky_bug.zip)


RendererRD::MaterialStorage *material_storage = RendererRD::MaterialStorage::get_singleton();
	RID m_src;
	m_src = ginstance->data->material_override.is_valid() ? ginstance->data->material_override : p_material;
	SceneShaderForwardMobile::MaterialData *material = nullptr;
	if (m_src.is_valid()) {
		material = static_cast<SceneShaderForwardMobile::MaterialData *>(material_storage->material_get_data(m_src, RendererRD::MaterialStorage::SHADER_TYPE_3D));
		if (!material || !material->shader_data->valid) {
			material = nullptr;
		}
	}
	if (material) {
		if (ginstance->data->dirty_dependencies) {
			material_storage->material_update_dependency(m_src, &ginstance->data->dependency_tracker);
		}
	} else {
		material = static_cast<SceneShaderForwardMobile::MaterialData *>(material_storage->material_get_data(scene_shader.default_material, RendererRD::MaterialStorage::SHADER_TYPE_3D));
		m_src = scene_shader.default_material;
	}
	ERR_FAIL_COND(!material);
	_geometry_instance_add_surface_with_material_chain(ginstance, p_surface, material, m_src, p_mesh);
	if (ginstance->data->material_overlay.is_valid()) {
		m_src = ginstance->data->material_overlay;
		material = static_cast<SceneShaderForwardMobile::MaterialData *>(material_storage->material_get_data(m_src, RendererRD::MaterialStorage::SHADER_TYPE_3D));
		if (material && material->shader_data->valid) {
			if (ginstance->data->dirty_dependencies) {
				material_storage->material_update_dependency(m_src, &ginstance->data->dependency_tracker);
			}
			_geometry_instance_add_surface_with_material_chain(ginstance, p_surface, material, m_src, p_mesh);