small fixes

[merklefruit]

Couple small cleanups found while reading part of the codebase for unrelated reasons.

• use alloy::hex::encode_prefixed where possible instead of format!("0x{}", hex::encode())
• avoid cloning pubkeys unnecessarily in BuilderApi::register_validators
• reverted helix docker tag to 0.1.0 for consistency. Remote versions are still unsupported so far.