Consolidate `--log-format json` and `--log-timestamps` options

[geo2a]

When we give both --log-format json and --log-timestamps options, we get logs that look like this:

2024-05-23T15:03:23:.703160 {"context":["booster","execute",{"term":"f76c7c3"},{"rewrite":"157d9251db4c252c841d0e6b2a2cded8737142b5719012bd6ecd9b5f7152cb55"},"detail"],"message":"...kproj/evm-semantics/evm.md :  (909, 10)"}

We should instead include the timestamp into the JSON object itself.