Fix temporary key setting in filter.epi_archive that has to be reset

[brookslogan]

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Change explanations for reviewer

filter.epi_archive sets a data.table's key before feeding it into new_epi_archive, thinking that / planning for new_epi_archive to require the key to already be set properly. But it did it improperly. Thankfully, new_epi_archive actually just does a second round of sorting if required. There are some warnings in some versions of data.table in some cases from the bogus intermediate key.

The archive key ordering is still a bit weird... I thought I already completed most of the geo-time-other[-version] --> geo-other-time[-version] changes, but apparently not. Not adjusting this now because example data sets need checked.

Don't think this is worth a version bump or changelog entry.

Magic GitHub syntax to mark associated Issue(s) as resolved when this is merged into the default branch

• Resolves #{issue number}

[brookslogan]

/style

[brookslogan]

This seems like a straightforward fix, and it addresses potential warnings that might start cropping up in other PRs. Just going to merge.