cmake: Clean up macOS install target

cmake: Clean up macOS install target

- Stop hardcoding CMAKE_INSTALL_PREFIX to CMAKE_BINARY_DIR so install
  products are in a predictable location
- Use install/fixup_bundle to create stand-alone app bundles for cube,
  cubepp, vulkaninfo-bundle at CMAKE_INSTALL_PREFIX
- Set INSTALL_RPATH_USE_LINK_PATH for bundle targets so fixup_bundle can
  find libraries when run from the install tree
- Remove rpath from install copy of vulkaninfo
- Remove extraneous copy of libMoltenVK.dylib from CMAKE_INSTALL_PREFIX/demos
- Update BUILD.md