Allow core-eval to update Zoe's choice of ZCF bundle when starting new contracts

[Chris-Hibbert]

What is the Problem Being Solved?

The version of the ZCF bundle that Zoe uses when starting a new contract is a hard-coded string in Zoe, which is looked up in a immutable kernel mapping table. In order to migrate to a new ZCF (#7900), something has to change.

Description of the Design

Some investigation required.

Security Considerations

Zoe isn't a contract, so it can't be subject to contract governance. It still needs to be legible that the ZCF can be changed, and what the current state is.

Scaling Considerations

Not applicable.

Test Plan

show, in a bootstrap test, that ZCF can be updated and new contracts can make use of it.