feat(`anvil`): fully offline mode for `anvil` fork

### Component

Anvil

### Describe the feature you would like

It does not seem like it's currently possible to do a fully offline version of an anvil fork. Even with the following startup commands, it still tries to reach out to the RPC:

```
anvil --fork-url https://sepolia.base.org --optimism --fork-block-number 20702367 --fork-chain-id 84532 --host 0.0.0.0 --load-state ~/state.json --no-mining
```

(I ensured that both state.json and storage.json exist for that fork number).

when attempting to call `eth_chainId`

I get
```
Error: failed to get fork block

Caused by:
0: error sending request for url (https://sepolia.base.org/)
1: error sending request for url (https://sepolia.base.org/)
2: operation timed out
```

(the env does not have access to the internet)

### Additional context

_No response_