fix(ci): unignore tracked cargo config

[okjodom]

Summary

• stop ignoring the tracked .cargo/config.toml file
• keep the rest of the .cargo directory ignored

Why

• release-plz aborts when a tracked file is also ignored
• the current .gitignore causes .cargo/config.toml to appear as an uncommitted ignored change in CI

Testing

• verified git no longer reports .cargo/config.toml as ignored with git check-ignore
• not run locally

[Copilot]

Copilot wasn't able to review any files in this pull request.

---

💡 Add Copilot custom instructions for smarter, more guided reviews. Learn how to get started.