### 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)


RendererRD::MaterialStorage::split_double(inst->transform.origin.y, &push_constant.transform[13], &push_constant.transform[7]);
			RendererRD::MaterialStorage::split_double(inst->transform.origin.z, &push_constant.transform[14], &push_constant.transform[11]);
#endif
		} else {
			RendererRD::MaterialStorage::store_transform(Transform3D(), push_constant.transform);
		}
		push_constant.flags = inst->flags_cache;
		push_constant.gi_offset = inst->gi_offset_cache;
		push_constant.layer_mask = inst->layer_mask;
		push_constant.instance_uniforms_ofs = uint32_t(inst->shader_uniforms_offset);
		if (p_params->pass_mode == PASS_MODE_DEPTH_MATERIAL) {
			
			push_constant.lightmap_uv_scale[0] = p_params->uv_offset.x;
			push_constant.lightmap_uv_scale[1] = p_params->uv_offset.y;
		} else {
			push_constant.lightmap_uv_scale[0] = inst->lightmap_uv_scale.position.x;
			push_constant.lightmap_uv_scale[1] = inst->lightmap_uv_scale.position.y;
			push_constant.lightmap_uv_scale[2] = inst->lightmap_uv_scale.size.x;
			push_constant.lightmap_uv_scale[3] = inst->lightmap_uv_scale.size.y;
		};
		RID material_uniform_set;
		SceneShaderForwardMobile::ShaderData *shader;
		void *mesh_surface;
		if (shadow_pass) {
			material_uniform_set = surf->material_uniform_set_shadow;
			shader = surf->shader_shadow;
			mesh_surface = surf->surface_shadow;
		} else {
			if (inst->use_projector) {
				base_spec_constants |= 1 << SPEC_CONSTANT_USING_PROJECTOR;