make_read_only_recursively: don't touch symlink targets

This fixes a problem visible when installing openssl 1.1.1d from component/openssl. At the end of install step, some symlinks are created inside openssl install directory for all certificates from ca-certificates part, then the "make read only" step occurs, which recursively chmod on files and folder in openssl installed parts, but since this was following symlinks, it was also trying to make readonly in ca-certificates parts, which in my case was belonging to another unix user, because it was a shared part folder from outer level slapos.