### 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 (ginstance->data->cast_double_sided_shadows) {
		flags |= GeometryInstanceSurfaceDataCache::FLAG_USES_DOUBLE_SIDED_SHADOWS;
	}
	if (has_alpha || p_material->shader_data->depth_draw == SceneShaderForwardMobile::ShaderData::DEPTH_DRAW_DISABLED || p_material->shader_data->depth_test == SceneShaderForwardMobile::ShaderData::DEPTH_TEST_DISABLED) {
		
		flags |= GeometryInstanceSurfaceDataCache::FLAG_PASS_ALPHA;
		if ((p_material->shader_data->uses_depth_prepass_alpha || p_material->shader_data->uses_alpha_antialiasing) && !(p_material->shader_data->depth_draw == SceneShaderForwardMobile::ShaderData::DEPTH_DRAW_DISABLED || p_material->shader_data->depth_test == SceneShaderForwardMobile::ShaderData::DEPTH_TEST_DISABLED)) {
			flags |= GeometryInstanceSurfaceDataCache::FLAG_PASS_DEPTH;
			flags |= GeometryInstanceSurfaceDataCache::FLAG_PASS_SHADOW;
		}
	} else {
		flags |= GeometryInstanceSurfaceDataCache::FLAG_PASS_OPAQUE;
		flags |= GeometryInstanceSurfaceDataCache::FLAG_PASS_DEPTH;
		flags |= GeometryInstanceSurfaceDataCache::FLAG_PASS_SHADOW;
	}
	if (p_material->shader_data->uses_particle_trails) {
		flags |= GeometryInstanceSurfaceDataCache::FLAG_USES_PARTICLE_TRAILS;
	}
	SceneShaderForwardMobile::MaterialData *material_shadow = nullptr;
	void *surface_shadow = nullptr;
	if (!p_material->shader_data->uses_particle_trails && !p_material->shader_data->writes_modelview_or_projection && !p_material->shader_data->uses_vertex && !p_material->shader_data->uses_discard && !p_material->shader_data->uses_depth_prepass_alpha && !p_material->shader_data->uses_alpha_clip && !p_material->shader_data->uses_alpha_antialiasing) {
		flags |= GeometryInstanceSurfaceDataCache::FLAG_USES_SHARED_SHADOW_MATERIAL;
		material_shadow = static_cast<SceneShaderForwardMobile::MaterialData *>(RendererRD::MaterialStorage::get_singleton()->material_get_data(scene_shader.default_material, RendererRD::MaterialStorage::SHADER_TYPE_3D));
		RID shadow_mesh = mesh_storage->mesh_get_shadow_mesh(p_mesh);
		if (shadow_mesh.is_valid()) {
			surface_shadow = mesh_storage->mesh_get_surface(shadow_mesh, p_surface);
		}
	} else {
		material_shadow = p_material;
	}