`follow-chain` does not work with child chains. (Testnet Babbage)

[afck]

The backport of https://github.com/linera-io/linera-protocol/pull/3887 fails on testnet_babbage.

The main problem seems to be that we try to use "local" validators (i.e. the ones in the to-be-synchronized chain's state) in several places, and these are not defined for the uninitialized chain yet.