When migrating files between storage engines with "bin/files migrate ...", skip expired temporary files

Summary:
See T7148. This just cheats us out of a weird sort of race where we:

• Dump an instance, including some F123 which is a temporary file which expires in 3 minutes.
• A few minutes later, the daemons delete the data for that file.
• A few minutes after that, we try to bin/files migrate --copy to copy the data from S3 into the MySQL blob store.
• This fails since the data is already gone.

Instead, just skip these files since they're already dead to us.

Test Plan: Faked this locally, will migrate the PHI769 instance on aux001.

Reviewers: amckinley

Reviewed By: amckinley

Differential Revision: https://secure.phabricator.com/D19536