### 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)


Color(0, 0, 0, 0),
			Color(0, 0, 0, 0)
		};
		RD::DrawListID draw_list = RD::get_singleton()->draw_list_begin(p_framebuffer, RD::INITIAL_ACTION_CLEAR, RD::FINAL_ACTION_READ, RD::INITIAL_ACTION_CLEAR, RD::FINAL_ACTION_READ, clear, 1.0, 0, p_region);
		const int uv_offset_count = 9;
		static const Vector2 uv_offsets[uv_offset_count] = {
			Vector2(-1, 1),
			Vector2(1, 1),
			Vector2(1, -1),
			Vector2(-1, -1),
			Vector2(-1, 0),
			Vector2(1, 0),
			Vector2(0, -1),
			Vector2(0, 1),
			Vector2(0, 0),
		};
		for (int i = 0; i < uv_offset_count; i++) {
			Vector2 ofs = uv_offsets[i];
			ofs.x /= p_region.size.width;
			ofs.y /= p_region.size.height;
			render_list_params.uv_offset = ofs;
			_render_list(draw_list, RD::get_singleton()->framebuffer_get_format(p_framebuffer), &render_list_params, 0, render_list_params.element_count); 
		}
		render_list_params.uv_offset = Vector2();
		_render_list(draw_list, RD::get_singleton()->framebuffer_get_format(p_framebuffer), &render_list_params, 0, render_list_params.element_count); 
		RD::get_singleton()->draw_list_end();
	}
	RD::get_singleton()->draw_command_end_label();
}
void RenderForwardMobile::_render_sdfgi(Ref<RenderSceneBuffersRD> p_render_buffers, const Vector3i &p_from, const Vector3i &p_size, const AABB &p_bounds, const PagedArray<RenderGeometryInstance *> &p_instances, const RID &p_albedo_texture, const RID &p_emission_texture, const RID &p_emission_aniso_texture, const RID &p_geom_facing_texture, float p_exposure_normalization) {