cmake: Update CMake to check env for MoltenVK
cmake: Update CMake to check env for MoltenVK

Updated CMake files to use MOLTENVK_REPO_ROOT as an
environment variable if no CMake argument is provided.
CMake argument takes priority over environment variable.

Change-Id: I661a56f7bc6c8c43fe4d4f7013e26dde87fb41e4

KhronosGroup/Vulkan-Tools,KhronosGroup/Vulkan-Tools,KhronosGroup/Vulkan-Tools,KhronosGroup/Vulkan-Tools