### 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)


ERR_FAIL_INDEX(p_id, (RendererRD::ForwardID)forward_id_allocators[p_type].allocations.size());
	forward_id_allocators[p_type].allocations[p_id] = false;
}
void RenderForwardMobile::ForwardIDStorageMobile::map_forward_id(RendererRD::ForwardIDType p_type, RendererRD::ForwardID p_id, uint32_t p_index) {
	forward_id_allocators[p_type].map[p_id] = p_index;
}
void RenderForwardMobile::ForwardIDStorageMobile::fill_push_constant_instance_indices(GeometryInstanceForwardMobile::PushConstant *p_push_constant, uint32_t &spec_constants, const GeometryInstanceForwardMobile *p_instance) {
	
	p_push_constant->omni_lights[0] = 0xFFFFFFFF;
	p_push_constant->omni_lights[1] = 0xFFFFFFFF;
	p_push_constant->spot_lights[0] = 0xFFFFFFFF;
	p_push_constant->spot_lights[1] = 0xFFFFFFFF;
	p_push_constant->decals[0] = 0xFFFFFFFF;
	p_push_constant->decals[1] = 0xFFFFFFFF;
	p_push_constant->reflection_probes[0] = 0xFFFFFFFF;
	p_push_constant->reflection_probes[1] = 0xFFFFFFFF;
	if (p_instance->omni_light_count == 0) {
		spec_constants |= 1 << SPEC_CONSTANT_DISABLE_OMNI_LIGHTS;
	}
	if (p_instance->spot_light_count == 0) {
		spec_constants |= 1 << SPEC_CONSTANT_DISABLE_SPOT_LIGHTS;
	}
	if (p_instance->reflection_probe_count == 0) {
		spec_constants |= 1 << SPEC_CONSTANT_DISABLE_REFLECTION_PROBES;
	}
	if (p_instance->decals_count == 0) {
		spec_constants |= 1 << SPEC_CONSTANT_DISABLE_DECALS;