Don't ship /var/cache/man/* in the ISO

Originally created by @intrigeri on #9417 (Redmine)

Not only this directory needlessly takes room in ISO images, but it also makes IUKs bigger than they could be (since there are time-based variations in there). live-build 5.x has a hook that does rm -rf /var/cache/man/*.

Feature Branch: bugfix/9417-no-var-cache-man

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