tar: fixed precision of PRUNE_BACKUPS_DAYS to correctly prune at the day value given

[itzg]

It turns out the precision of -mtime is a bit fuzzy where a value of 1 ends up matching files older than 2 days:

When find
figures out how many 24-hour periods ago the file was last accessed, any fractional
part is ignored, so to match -atime +1, a file has to have been accessed at least
two days ago.

Using -mmin instead, which now also allows for fractional values of PRUNE_BACKUPS_DAYS

Reported in Discord