sysupdate: Allow a partial version to be vacuumed

sysupdate: Allow a partial version to be vacuumed

Previously we prevented partial and pending versions from being
vacuumed. But until we support resuming downloads, there’s nothing else
which can be done with a partial version except to vacuum it and try
again.

Accordingly, allow partial versions (but not pending versions) to be
vacuumed.

This behaviour can be changed again once resuming downloads is supported
— at that point I expect we’ll want to try resuming the partial download
rather than throwing it all away and trying again.

Signed-off-by: Philip Withnall <pwithnall@gnome.org>
Helps: https://github.com/systemd/systemd/issues/41502