mkosi: only print the summary when explicitly requested
mkosi: only print the summary when explicitly requested

Originally, the summary was printed whenever a build was done.
With 1f9a758fad3debbdddd2947a19978de3909aabf7, it is only printed
for cached builds. But that is surprising: when I turn on caching,
I suddently get extra output. Let's just print the summary when
requested.

systemd/mkosi,systemd/mkosi