possible race condition with the apt.cron.daily job

Binary package hint: update-manager

There is a possible race condition when update-manager is run with the daily apt cron job. When "apt-get check" is run it in the cron script to take the lock and rebuild the caches, that may happen between two dpkg invocations in the package installation code.

The fix must:
a) add a workaround during the release upgrade to make sure the apt.cron.daily job is not run (either via a custom config in /etc/apt/apt.conf.d) or by disabling anacron/cron during the upgrade
b) add code into the apt cron job to test for the lock without actually taking it (or for a much shorter time than apt-get check that also rebuild the apt pkgcache.bin)