### 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)


SceneShaderForwardMobile::MaterialData *material = p_material;
	RendererRD::MaterialStorage *material_storage = RendererRD::MaterialStorage::get_singleton();
	_geometry_instance_add_surface_with_material(ginstance, p_surface, material, p_mat_src.get_local_index(), material_storage->material_get_shader_id(p_mat_src), p_mesh);
	while (material->next_pass.is_valid()) {
		RID next_pass = material->next_pass;
		material = static_cast<SceneShaderForwardMobile::MaterialData *>(material_storage->material_get_data(next_pass, RendererRD::MaterialStorage::SHADER_TYPE_3D));
		if (!material || !material->shader_data->valid) {
			break;
		}
		if (ginstance->data->dirty_dependencies) {
			material_storage->material_update_dependency(next_pass, &ginstance->data->dependency_tracker);
		}
		_geometry_instance_add_surface_with_material(ginstance, p_surface, material, next_pass.get_local_index(), material_storage->material_get_shader_id(next_pass), p_mesh);
	}
}
void RenderForwardMobile::_geometry_instance_add_surface(GeometryInstanceForwardMobile *ginstance, uint32_t p_surface, RID p_material, RID p_mesh) {