Have check-mirrors download to TMPDIR instead of PWD

Currenly check-mirror always use $PWD for downloads. It should instead download TMPDIR (using standard mktmp facilities) by default.

Source code: git clone https://git.tails.boum.org/check-mirrors
Mentoring: tails-mirrors@boum.org

Original created by @sajolida on 8426 (Redmine)

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