### 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)


return 256;
}
RenderForwardMobile *RenderForwardMobile::singleton = nullptr;
void RenderForwardMobile::_update_shader_quality_settings() {
	Vector<RD::PipelineSpecializationConstant> spec_constants;
	RD::PipelineSpecializationConstant sc;
	sc.type = RD::PIPELINE_SPECIALIZATION_CONSTANT_TYPE_INT;
	sc.constant_id = SPEC_CONSTANT_SOFT_SHADOW_SAMPLES;
	sc.int_value = soft_shadow_samples_get();
	spec_constants.push_back(sc);
	sc.constant_id = SPEC_CONSTANT_PENUMBRA_SHADOW_SAMPLES;
	sc.int_value = penumbra_shadow_samples_get();
	spec_constants.push_back(sc);
	sc.constant_id = SPEC_CONSTANT_DIRECTIONAL_SOFT_SHADOW_SAMPLES;
	sc.int_value = directional_soft_shadow_samples_get();
	spec_constants.push_back(sc);
	sc.constant_id = SPEC_CONSTANT_DIRECTIONAL_PENUMBRA_SHADOW_SAMPLES;
	sc.int_value = directional_penumbra_shadow_samples_get();
	spec_constants.push_back(sc);
	sc.type = RD::PIPELINE_SPECIALIZATION_CONSTANT_TYPE_BOOL;
	sc.constant_id = SPEC_CONSTANT_DECAL_USE_MIPMAPS;
	sc.bool_value = decals_get_filter() == RS::DECAL_FILTER_NEAREST_MIPMAPS ||
			decals_get_filter() == RS::DECAL_FILTER_LINEAR_MIPMAPS ||
			decals_get_filter() == RS::DECAL_FILTER_NEAREST_MIPMAPS_ANISOTROPIC ||
			decals_get_filter() == RS::DECAL_FILTER_LINEAR_MIPMAPS_ANISOTROPIC;
	spec_constants.push_back(sc);
	sc.constant_id = SPEC_CONSTANT_PROJECTOR_USE_MIPMAPS;
	sc.bool_value = light_projectors_get_filter() == RS::LIGHT_PROJECTOR_FILTER_NEAREST_MIPMAPS ||
			light_projectors_get_filter() == RS::LIGHT_PROJECTOR_FILTER_LINEAR_MIPMAPS ||
			light_projectors_get_filter() == RS::LIGHT_PROJECTOR_FILTER_NEAREST_MIPMAPS_ANISOTROPIC ||