fix(config): normalize deny_warnings from env vars

[aso20455]

normalize_defaults uses self.deny_warnings to check if the deprecated deny_warnings flag was set, but self here is always Config::default() (where deny_warnings = false), so the condition is dead code.

The TOML path works fine because BackwardsCompatTomlProvider converts the deny_warnings key to deny in the dict directly. But env vars like FOUNDRY_DENY_WARNINGS=true bypass that provider and rely on normalize_defaults to do the conversion — which never triggers.

The fix reads deny_warnings from the figment, same pattern already used for solc/evm_version a few lines above in the same function.

Introduced in c7d9703 (#11445).