build: Updated BUILD.md for spirv-tools changes
build: Updated BUILD.md for spirv-tools changes

Change-Id: Ic2f62a226474dbe5705be2706efc093b3dde7b9e

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