Backport !1058 “packagekit: Ensure update is prepared when all packages are downloaded” to gnome-41

Having all packages for update downloaded doesn't mean the update is prepared. As the "all downloaded" state is used to recognize that difference mark one package for a download, thus the update created the required prepared-update file, when it does not exist.

Closes #1114 (closed)

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Trivial backport of !1058 (merged) to gnome-41.