When restoring a backup, discourse decompresses the backup archive in
the /share/discourse/tmp dir. Before this change, it is linked to /run
which is typically backed by memory, so the backup will fail to
restore if you do not have enough memory on your system to contain the
backup. This has already happened to me on two small forums.
This moves tmp to the StateDirectory /var/lib/discourse/tmp which is
typically backed by disk.
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