[frontend] make clean_old_builds query join() explicit
    
    The python3-sqlalchemy-1.3.1 on Fedora 30+ is more scrict on
    join()s, and it better detected our over-linkage between
    CoprDir--Build--Package tables (the broken schema problem is
    tracked as tracked as issue #617).
    
    Detected by testsuite on F30:
    E sqlalchemy.exc.InvalidRequestError: Can't determine which FROM
    clause to join from, there are multiple FROMS which can join to
    this entity. Try adding an explicit ON clause to help resolve the
    ambiguity.
    
    Fixes: PR#666