if `FORK_URL` is defined, use that as url

[8times4]

Hey Team,

I've had some issues running this by default for Base, since no matter if the FORK_URL was defined, it'd simply get CHAIN_ID_NOT_SUPPORTED error as chain_id_to_fork_url function does not have Base statically added.

I'm proposing this fix, since when passing FORK_URL in, you're already constrained to use that chain for simulation, else you get INCORRECT_CHAIN_ID, so it just allows the usage of more than the static chains.

Best, 8x4