[UE-90771] M_Volumetric_Sky_Prototype Material has compile errors

The material “M_Volumetric_Sky_Prototype” has numerous compile errors that prevent it from being used.

This was reported and tested in 4.25 Preview 2 (CL-12055273) and reproduced in 4.25 Preview 3 (CL 2027741)

See: Unreal Engine Issues and Bug Tracker (UE-90771)