Bug fix on the cp -R command that copy in an inconsistent way

The last instruction of compile is "cp -R", and it was copying /app/vendor/R into $VENDOR_DIR/R. It's not a correct behaviour, because the R folder become a subfolder of $VENDOR_DIR/R, so the newly installed packages can't be found.

BTW thank you @virtualstaticvoid for your amazing work on this buildpack!