spec: Test for /var being writable before making /var/log/journal
    
    rpm-ostree has `/var` be read-only during package installs, because
    a whole part of the "transactional update" model is that your system's
    data stays untouched, and `/var` is system data.
    
    See e.g. https://src.fedoraproject.org/rpms/mock/pull-request/2
    and the tracker https://bugzilla.redhat.com/show_bug.cgi?id=1352154
    
    Just to squash some error spew during tree composes.