don't lock dpkg in update commands

The update command acquires a lock on lists/, but at the end it will
also require the dpkg/lock while building the binary caches. That seems
rather pointless as we are only reading those files, not causing writing
in them. This can also cause problems if a package installation is
running and a background process (like cron) starts an update: If you
are "lucky" enough the update process will pick the dpkg lock in between
apt calls causing the installation process to fail.

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