### 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)


RS::InstanceType type = RSG::utilities->get_base_type(p_base);
	ERR_FAIL_COND_V(!((1 << type) & RS::INSTANCE_GEOMETRY_MASK), nullptr);
	GeometryInstanceForwardMobile *ginstance = geometry_instance_alloc.alloc();
	ginstance->data = memnew(GeometryInstanceForwardMobile::Data);
	ginstance->data->base = p_base;
	ginstance->data->base_type = type;
	ginstance->data->dependency_tracker.userdata = ginstance;
	ginstance->data->dependency_tracker.changed_callback = _geometry_instance_dependency_changed;
	ginstance->data->dependency_tracker.deleted_callback = _geometry_instance_dependency_deleted;
	ginstance->_mark_dirty();
	return ginstance;
}
void RenderForwardMobile::GeometryInstanceForwardMobile::set_use_lightmap(RID p_lightmap_instance, const Rect2 &p_lightmap_uv_scale, int p_lightmap_slice_index) {
	lightmap_instance = p_lightmap_instance;
	lightmap_uv_scale = p_lightmap_uv_scale;
	lightmap_slice_index = p_lightmap_slice_index;
	_mark_dirty();
}
void RenderForwardMobile::GeometryInstanceForwardMobile::set_lightmap_capture(const Color *p_sh9) {