backtest-build-block relies on coinbase_secret_key which is not set in backtest-config

As per the title, when running backtest-build-block we rely on having a coinbase signer here: https://github.com/flashbots/rbuilder/blob/develop/crates/rbuilder/src/backtest/backtest_build_block.rs#L112

Which ultimately needs a coinbase_secret_key being set to derive that from:
https://github.com/flashbots/rbuilder/blob/develop/crates/rbuilder/src/live_builder/base_config.rs#L225

But it's not present in backtesting. That means that with the example backtesting config we throw an error when trying to get the coinbase signer as an input to build the block.

If we're backtesting and there is no key set, we could generate a random one as a simple fix.