Ambient Cubemap Assignment in Post Process Volume does not work with Forward Renderer

The Ambient Cubemap assignment in a Post Process Volume doesn't work when using the Forward Renderer.

I want to note that Cubemap assignment in the Skylight does work. I checked this just to make sure that this was actually supported for the Forward Renderer at this point.

Tested in:
Broken: 4.14 Preview 2 Binary CL-3179539
Broken: 4.15 Dev-Main CL-3189774

1. Open a project with Forward Shading enabled via the Project Settings.
2. Add a Post Process Volume to the scene (make it unbound for ease).
3. In the PPV's Ambient Cubemap section assign a Cubemap.

Regression: No

Results: Ambient Cubemap doesn't work when assigned to a post process volume with Forward Rendering Enabled.