Fix ignore venv in build_directory_md.py

[FishyGitHubUser]

Describe your change:

Fixes #8736 - Ignores virtual environment named venv to prevent validate_filenames from reading it, and failing.

• Add an algorithm?
• Fix a bug or typo in an existing algorithm?
• Add or change doctests? -- Note: Please avoid changing both code and tests in a single pull request.
• Documentation change?

Checklist:

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• This pull request is all my own work -- I have not plagiarized.
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[FishyGitHubUser]

The added condition's compare value "venv" should have a standardized name, so anyone running a virtual environment will not get the same issue.