fix: examples

[DaniPopes]

Motivation

fixes #2151

Solution

• abigen example: write bindings env::tmpdir and clear if it exists already
• gas oracle: create a client even if no api_key env is set since it's not required; use Client::from_env
• call override: skip executing if geth fails / cannot be found; I tried substituting with Anvil but it doesn't seem to work, presumably because the call override feature is missing from Anvil? @mattsse

PR Checklist

• Added Tests
• Added Documentation
• Updated the changelog
• Breaking changes