### 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)


should_request_redraw = true;
		}
		
		SceneShaderForwardMobile::ShaderData::CullVariant cull_variant;
		if (p_params->pass_mode == PASS_MODE_DEPTH_MATERIAL || ((p_params->pass_mode == PASS_MODE_SHADOW || p_params->pass_mode == PASS_MODE_SHADOW_DP) && surf->flags & GeometryInstanceSurfaceDataCache::FLAG_USES_DOUBLE_SIDED_SHADOWS)) {
			cull_variant = SceneShaderForwardMobile::ShaderData::CULL_VARIANT_DOUBLE_SIDED;
		} else {
			bool mirror = surf->owner->mirror;
			if (p_params->reverse_cull) {
				mirror = !mirror;
			}
			cull_variant = mirror ? SceneShaderForwardMobile::ShaderData::CULL_VARIANT_REVERSED : SceneShaderForwardMobile::ShaderData::CULL_VARIANT_NORMAL;
		}
		RS::PrimitiveType primitive = surf->primitive;
		RID xforms_uniform_set = surf->owner->transforms_uniform_set;
		SceneShaderForwardMobile::ShaderVersion shader_version = SceneShaderForwardMobile::SHADER_VERSION_MAX; 
		switch (p_params->pass_mode) {
			case PASS_MODE_COLOR:
			case PASS_MODE_COLOR_TRANSPARENT: {
				if (element_info.uses_lightmap) {
					shader_version = p_params->view_count > 1 ? SceneShaderForwardMobile::SHADER_VERSION_LIGHTMAP_COLOR_PASS_MULTIVIEW : SceneShaderForwardMobile::SHADER_VERSION_LIGHTMAP_COLOR_PASS;
				} else {
					shader_version = p_params->view_count > 1 ? SceneShaderForwardMobile::SHADER_VERSION_COLOR_PASS_MULTIVIEW : SceneShaderForwardMobile::SHADER_VERSION_COLOR_PASS;
				}
			} break;
			case PASS_MODE_SHADOW: {
				shader_version = p_params->view_count > 1 ? SceneShaderForwardMobile::SHADER_VERSION_SHADOW_PASS_MULTIVIEW : SceneShaderForwardMobile::SHADER_VERSION_SHADOW_PASS;
			} break;
			case PASS_MODE_SHADOW_DP: {
				ERR_FAIL_COND_MSG(p_params->view_count > 1, "Multiview not supported for shadow DP pass");