### 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 (inst->use_soft_shadow) {
				base_spec_constants |= 1 << SPEC_CONSTANT_USING_SOFT_SHADOWS;
			}
			forward_id_storage_mobile->fill_push_constant_instance_indices(&push_constant, base_spec_constants, inst);
#ifdef DEBUG_ENABLED
			if (unlikely(get_debug_draw_mode() == RS::VIEWPORT_DEBUG_DRAW_LIGHTING)) {
				material_uniform_set = scene_shader.default_material_uniform_set;
				shader = scene_shader.default_material_shader_ptr;
			} else if (unlikely(get_debug_draw_mode() == RS::VIEWPORT_DEBUG_DRAW_OVERDRAW)) {
				material_uniform_set = scene_shader.overdraw_material_uniform_set;
				shader = scene_shader.overdraw_material_shader_ptr;
			} else if (unlikely(get_debug_draw_mode() == RS::VIEWPORT_DEBUG_DRAW_PSSM_SPLITS)) {
				material_uniform_set = scene_shader.debug_shadow_splits_material_uniform_set;
				shader = scene_shader.debug_shadow_splits_material_shader_ptr;
			} else {
#endif
				material_uniform_set = surf->material_uniform_set;
				shader = surf->shader;
				surf->material->set_as_used();
#ifdef DEBUG_ENABLED
			}
#endif
			mesh_surface = surf->surface;
		}
		if (!mesh_surface) {
			continue;
		}
		
		if (shader->uses_time) {