cross-busybox.xml: artifact busybox binary in build directory

- Don't artifact the copy of `busybox` located in `$WORKSPACE`, only
  artifact what's in the build directory.
  - This means that the ARM and PPC builds will get artifacted
    separately, instead of both `busybox` binaries in the same tarball
  - Both `busybox` binaries are still copied into `$WORKSPACE`