spec: always make debuginfo cache setgid

libreport relies on the directory being both setuid and setgid since
commit 82ab820df1e407bb91facc351a96bb38598f91f7. Debuginfo download
won't work with any newer libreport if the cache does not have the
setgid bit set.

Signed-off-by: Martin Milata <mmilata@redhat.com>
Signed-off-by: Jakub Filak <jfilak@redhat.com>