Before #5150, this section of the multi-user installer used rsync and was (IIRC) ~idempotent:

Dropping the rsync dependency to support debian makes sense, but it would be nice to figure out whether we can portably make this step idempotent again--and do it, if it's simple enough.

Shooting from the hip:

• this might be as simple as using cp's -f flag, if it's portable?
• it's probably not as simple as just ignoring the exit code from cp, since there could be other issues
• it's probably not as simple as just diffing the directories, since an existing store will contain more than the store seed

(If we can't do this without, say, wiping the store or restoring the rsync dependency, we should probably figure out how to fail faster until we can find another approach.)