simulators/ethereum/engine: fix cancun bpo schedule error

[taratorio]

After a378713f9212e8b3728ffc0c95b6885356e7993f a few ethereum/engine (Cancun) tests that start up a secondary Geth node for devp2p interactions started failing with:

Unable to spawn a secondary client: invalid chain configuration: missing entry for fork "cancun" in blobSchedule

Had a look and it seemed like the ethereum/engine simulator loads up a static genesis file from simulators/ethereum/engine/init/genesis.json and applies Paris/Cancun-specific modifications to it at runtime.

This PR updates the Cancun config modifier to also include the default Cancun BPO schedule in order to fix the initialisation error from Geth.

[taratorio]

@fjl fyi

[fjl]

Thanks for debugging this. I didn't get around to it yet, so I'm happy you did!