checkapk: fix size difference output when multiple info matches

I rebuilt networkmanager locally and was greeted with this:

Size difference for networkmanager: 16 MiB -> 2772 KiB

It turns out networkmanager-elogind provides networkmanager so apk info will list both, and the former is much bigger so it looked like the rebuilt package was much smaller when there was almost no difference (as expected)

Fix by matching on exact package name for both lines; this is already what we were doing with the local package anyway and the apk fetch correctly picks the correct package in this case