mirrors: Sources.xz hash mismatch occurs during syncing

[ja5087]

See discussion on IRC

https://irclogs.ocf.berkeley.edu/rebuild/2020-02-23#412270

[jvperrin]

Nice, thanks for making this, it's definitely good to have somewhere to actually track this.

Move discussion from March: https://irclogs.ocf.berkeley.edu/rebuild/2020-03-09#430245;

And it seems to happen with -updates or -backports dists especially, likely because they get updates more frequently