### 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)


Vector<RID> fb;
	fb.push_back(p_color); 
	fb.push_back(p_depth); 
	
	Vector<RD::FramebufferPass> passes;
	RD::FramebufferPass pass;
	
	pass.color_attachments.push_back(0);
	pass.depth_attachment = 1;
	
	passes.push_back(pass);
	
	passes.push_back(pass);
	
	passes.push_back(pass);
	return RD::get_singleton()->framebuffer_create_multipass(fb, passes);
}
void RenderForwardMobile::setup_render_buffer_data(Ref<RenderSceneBuffersRD> p_render_buffers) {
	Ref<RenderBufferDataForwardMobile> data;
	data.instantiate();
	p_render_buffers->set_custom_data(RB_SCOPE_MOBILE, data);
}
bool RenderForwardMobile::free(RID p_rid) {
	if (RendererSceneRenderRD::free(p_rid)) {
		return true;
	}
	return false;
}
float RenderForwardMobile::_render_buffers_get_luminance_multiplier() {