cmake: Set CMAKE_INSTALL_PREFIX to "" so loader will be built with sys paths
cmake: Set CMAKE_INSTALL_PREFIX to "" so loader will be built with sys paths

CMake seems to default to non-null but for system SDK builds want this to
be empty string.

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