chore: add missing deps for glean-generate

[bbangert]

Because:

• glean-generate needs venv and pip to run correctly

This commit:

• Add missing Python venv/pip dependencies.

Checklist

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• [x] My commit is GPG signed.
• [x] If applicable, I have modified or added tests which pass locally.
• [x] I have added necessary documentation (if appropriate).
• [x] I have verified that my changes render correctly in RTL (if appropriate).