chore: Update .gitignore for change in flakeheaven

[aucampia]

Summary of changes

flakeheaven was changed to place its cache directory in the working directory, this change adds the flakeheaven cache directory to the .gitignore file.

Checklist

• Checked that there aren't other open pull requests for
  the same change.
• Added tests for any changes that have a runtime impact.
• Checked that all tests and type checking passes.
• Considered granting push permissions to the PR branch,
  so maintainers can fix minor issues and keep your PR up to date.

[coveralls]

Coverage: 90.854%. Remained the same when pulling 3b00b5c on aucampia:aucampia/20230519T1207-fix_gitignore into e103078 on RDFLib:main.

[aucampia]

Merging without review, as this only changes .gitignore.