fix: forge create to use config values for verify preflight check

[dghelm]

Motivation

As described in #6466 , forge create --verify could not be called using foundry.toml

Solution

Update the etherscan API key from the Config in the information that is checked in the preflight. This means if no ETHERSCAN_API_KEY is set as an env var or flag, the config files will be used as opposed to failing the preflight check.

Note

If a test is needed here, I might need my handheld. Very new to Rust, so also unsure if it's okay here to make verify mutable.

[mattsse]

cool, ty!