Only run libwbclient %pre on upgrade, not fresh install
    
    This seems to be breaking live image build tests because the
    script is running before `rm` is installed. I do not know why
    it didn't fail the tests run *on the update itself*, but it's
    now causing tests of subsequent updates to fail, which is a
    problem. Probably an ordering issue.
    
    AFAIK, this script is meant to clean up stuff from earlier
    versions of the package, so it's not relevant on fresh installs,
    and we can just skip it and avoid any problems in the fresh
    install case.