Add argument to zap older versions of packages from all chroots

This adds argument -o, --old-bins to the pmbootstrap zap command which will simply call apk -v cache clean in all the chroots. It's very useful to run it once in a while to free up some disk space.

Also moved the mismatch clean before zapping the chroots. This avoids re-creating the chroot just to perform the repo index after the clean.