### 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)


}
	
	for (int i = 0; i < (int)p_render_data->instances->size(); i++) {
		GeometryInstanceForwardMobile *inst = static_cast<GeometryInstanceForwardMobile *>((*p_render_data->instances)[i]);
		Vector3 center = inst->transform.origin;
		if (p_render_data->scene_data->cam_orthogonal) {
			if (inst->use_aabb_center) {
				center = inst->transformed_aabb.get_support(-near_plane.normal);
			}
			inst->depth = near_plane.distance_to(center) - inst->sorting_offset;
		} else {
			if (inst->use_aabb_center) {
				center = inst->transformed_aabb.position + (inst->transformed_aabb.size * 0.5);
			}
			inst->depth = p_render_data->scene_data->cam_transform.origin.distance_to(center) - inst->sorting_offset;
		}
		uint32_t depth_layer = CLAMP(int(inst->depth * 16 / z_max), 0, 15);
		uint32_t flags = inst->base_flags; 
		if (inst->non_uniform_scale) {
			flags |= INSTANCE_DATA_FLAGS_NON_UNIFORM_SCALE;
		}
		bool uses_lightmap = false;
		
		if (p_render_list == RENDER_LIST_OPAQUE) {
			if (inst->lightmap_instance.is_valid()) {
				int32_t lightmap_cull_index = -1;
				for (uint32_t j = 0; j < scene_state.lightmaps_used; j++) {
					if (scene_state.lightmap_ids[j] == inst->lightmap_instance) {
						lightmap_cull_index = j;
						break;