USE_BASE_CONSENSUS not set in .env.mainnet and .env.sepolia, causing op-node to be used instead of base-consensus

[Sertug17]

The .env file sets USE_BASE_CONSENSUS=true, but .env.mainnet and .env.sepolia don't include this variable at all.

Since docker-compose.yml defaults to false:
USE_BASE_CONSENSUS=${USE_BASE_CONSENSUS:-false}

Operators running with NETWORK_ENV=.env.sepolia will silently use op-node instead of base-consensus, and will fail to support the Base V1 upgrade on April 20th.

Fix: add USE_BASE_CONSENSUS=true to both .env.mainnet and .env.sepolia.