When unattended-upgrades is run with the --dry-run flag, it still acquires a
cache lock (presumably from apt update?), and makes a pid file. These can
interfere with unattended-upgrade runs that are done without the --dry-run, and
can result in Cache acquire lock errors.

Ideally, for --dry-run, unattended-upgrades could use a temporary directory for
the lists location, and the pid, if it even needs one.

thanks!