Create a public repo for check-mirrors

Originally created by @sajolida on #7667 (Redmine)

Our mirror checking script, check-mirrors, has been kept in a private
repo so far. According to tails/tails#7653, this should be changed. We need to
create a public Git repo without the secret extra mirrors from the
current repo.

Related issues

• Related to tails/tails#8054
• Related to tails/tails#7666