fix(verify): improve etherscan section parsing

[mattsse]

Motivation

closes #3861
Closes #4571
closes #4563

This includes a fix that gives the etherscan_api_key of the config precedence over the chain. This is sound because, if --etherscan-api-key is provided via CLI it should be prefered over the etherscan table.

This also improves the error message instead of ETHERSCAN_API_KEY not found.

Failed to resolve env var `BSC_ETHERSCAN_API_KEY` in `${BSC_ETHERSCAN_API_KEY}`: environment variable not found

Context:
- environment variable not found

ptal @DaniPopes

Solution