message: Fix mvfst build directory
  Summary:
  According to `build_helper.sh` the `MVFST_BUILD_DIR` is `_build/build`.
  Pull Request resolved:
diff: @@ -16,7 +16,7 @@ trap 'cd $start_dir' EXIT
  # Must execute from the directory containing this script
  cd "$(dirname "$0")"
  -cd _build
  +cd _build/build
  # Uninstall is expected to fail the first time
  sudo make uninstall || true
  sudo make install
  