Use the mtime instead of the ctime to decide on the latest .changes file.

In some cases where both _arch.changes and _source.changes are present,
their ctime might be the same if they were copied out of the chroot at
the same time (same `mv` call, for example).

Use the mtime instead, which should be more realiable in our case, where
we don't actually care about last changes to the file attributes.

Signed-off-by: Mattia Rizzolo <mattia@debian.org>
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pages

00:01:49

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test

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