Multiple network flags should prevent the BN to start.

[nalepae]

Describe the bug

Let's run the BN with both --sepolia and --holesky flags, like:

bazel run //cmd/beacon-chain --config=release -- --execution-endpoint=http://localhost:8551 --sepolia --jwt-secret /Users/manu/OffchainLabs/ethereum/jwt.hex --checkpoint-sync-url https://sepolia.beaconstate.info --genesis-beacon-api-url https://sepolia.beaconstate.info --accept-terms-of-use --holesky --force-clear-db

The BN starts. It should exit on error immediately.

Has this worked before in a previous version?

Probably not.

Related to: https://github.com/prysmaticlabs/prysm/issues/13799