bodhi / pungi: disallow pkg reuse in updates pungi runs
    
    This should prevent a recent issue where a unsigned package managed to
    get synced out. The package was mistagged, was fixed, but then it's
    cached signed copies had been garbage collected. This caused the push to
    fail, but then the resumed one didn't regather the signed version of
    that package, and instead just reused the unsigned one.
    Setting this should prevent pungi from doing any package reuse here,
    which is fine as updates don't reuse packages.
    See https://pagure.io/pungi/issue/1480 for more information.
    
    Signed-off-by: Kevin Fenzi <kevin@scrye.com>