### 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)


GeometryInstanceSurfaceDataCache *sdcache = geometry_instance_surface_alloc.alloc();
	sdcache->flags = flags;
	sdcache->shader = p_material->shader_data;
	sdcache->material = p_material;
	sdcache->material_uniform_set = p_material->uniform_set;
	sdcache->surface = mesh_storage->mesh_get_surface(p_mesh, p_surface);
	sdcache->primitive = mesh_storage->mesh_surface_get_primitive(sdcache->surface);
	sdcache->surface_index = p_surface;
	if (ginstance->data->dirty_dependencies) {
		RSG::utilities->base_update_dependency(p_mesh, &ginstance->data->dependency_tracker);
	}
	
	sdcache->shader_shadow = material_shadow->shader_data;
	sdcache->material_uniform_set_shadow = material_shadow->uniform_set;
	sdcache->surface_shadow = surface_shadow ? surface_shadow : sdcache->surface;
	sdcache->owner = ginstance;
	sdcache->next = ginstance->surface_caches;
	ginstance->surface_caches = sdcache;
	
	sdcache->sort.sort_key1 = 0;
	sdcache->sort.sort_key2 = 0;
	sdcache->sort.surface_index = p_surface;
	sdcache->sort.material_id_low = p_material_id & 0x0000FFFF;
	sdcache->sort.material_id_hi = p_material_id >> 16;
	sdcache->sort.shader_id = p_shader_id;
	sdcache->sort.geometry_id = p_mesh.get_local_index();
	
	sdcache->sort.priority = p_material->priority;
}
void RenderForwardMobile::_geometry_instance_add_surface_with_material_chain(GeometryInstanceForwardMobile *ginstance, uint32_t p_surface, SceneShaderForwardMobile::MaterialData *p_material, RID p_mat_src, RID p_mesh) {