Block headers returning incorrect public key.

**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.