Block headers returning incorrect public key.

[mcdee]

Describe the bug

The spec at https://ethereum.github.io/builder-specs/#/Builder/getHeader states that the pubkey field in the message struct is the public key of the builder, however the public key of the relay is being returned in this field.

Steps to reproduce

1. Request a header with /eth/v1/builder/header/{slot}/{parent_hash}/{pubkey}
2. Check the pubkey field of the response

Expected behavior

The pubkey field should be the public key of the builder, not the relay.