### 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)


if (p_sh9) {
		if (lightmap_sh == nullptr) {
			lightmap_sh = RenderForwardMobile::get_singleton()->geometry_instance_lightmap_sh.alloc();
		}
		memcpy(lightmap_sh->sh, p_sh9, sizeof(Color) * 9);
	} else {
		if (lightmap_sh != nullptr) {
			RenderForwardMobile::get_singleton()->geometry_instance_lightmap_sh.free(lightmap_sh);
			lightmap_sh = nullptr;
		}
	}
	_mark_dirty();
}
void RenderForwardMobile::geometry_instance_free(RenderGeometryInstance *p_geometry_instance) {
	GeometryInstanceForwardMobile *ginstance = static_cast<GeometryInstanceForwardMobile *>(p_geometry_instance);
	ERR_FAIL_COND(!ginstance);
	if (ginstance->lightmap_sh != nullptr) {
		geometry_instance_lightmap_sh.free(ginstance->lightmap_sh);
	}
	GeometryInstanceSurfaceDataCache *surf = ginstance->surface_caches;
	while (surf) {
		GeometryInstanceSurfaceDataCache *next = surf->next;
		geometry_instance_surface_alloc.free(surf);
		surf = next;
	}
	memdelete(ginstance->data);
	geometry_instance_alloc.free(ginstance);
}
uint32_t RenderForwardMobile::geometry_instance_get_pair_mask() {