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.

/reviewed-on !10