### 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)


RendererRD::MeshStorage *mesh_storage = RendererRD::MeshStorage::get_singleton();
	bool has_read_screen_alpha = p_material->shader_data->uses_screen_texture || p_material->shader_data->uses_depth_texture || p_material->shader_data->uses_normal_texture;
	bool has_base_alpha = p_material->shader_data->uses_alpha && (!p_material->shader_data->uses_alpha_clip || p_material->shader_data->uses_alpha_antialiasing);
	bool has_blend_alpha = p_material->shader_data->uses_blend_alpha;
	bool has_alpha = has_base_alpha || has_blend_alpha || has_read_screen_alpha;
	uint32_t flags = 0;
	if (p_material->shader_data->uses_sss) {
		flags |= GeometryInstanceSurfaceDataCache::FLAG_USES_SUBSURFACE_SCATTERING;
	}
	if (p_material->shader_data->uses_screen_texture) {
		flags |= GeometryInstanceSurfaceDataCache::FLAG_USES_SCREEN_TEXTURE;
	}
	if (p_material->shader_data->uses_depth_texture) {
		flags |= GeometryInstanceSurfaceDataCache::FLAG_USES_DEPTH_TEXTURE;
	}
	if (p_material->shader_data->uses_normal_texture) {
		flags |= GeometryInstanceSurfaceDataCache::FLAG_USES_NORMAL_TEXTURE;
	}