Add Zeth to the nil package

[akokoshn]

Short Summary

Add Zeth binary to the nil package.
For get it locally run

nix build .#zeth
./result/bin/zeth-ethereum

What Changes Were Made

• Add derivation zeth which downloads binary from github release
• Add zeth-ethereum executable to the nil package under /usr/bin

Checklist

• I have read and followed the Contributing Guide
• I have tested the changes locally

[notion-workspace]

Integrate Zeth as re execute verifier