Function build_encrypted_write_payload 

pub(crate) fn build_encrypted_write_payload(
    ciphertext: &[u8; 32],
    mac: &[u8; 32],
) -> [u8; 64]

Assemble the 64-byte payload (ciphertext || mac) that the driver expects in [atecc608b::AteccChannel::write_32_encrypted].