PR Summary

This is needed because of the way the micromamba provisioning works, or else anything installed in the mamba environment is not found.

I also added a comment explaining it so that it won't be accidentally removed in the future (or will if that requirement is dropped.)

Now that I know you can trigger workflow_dispatch runs from any branch, I checked this before opening this PR: https://github.com/matplotlib/matplotlib/actions/runs/3972489758

PR Checklist

Documentation and Tests

• [n/a] Has pytest style unit tests (and pytest passes)
• [n/a] Documentation is sphinx and numpydoc compliant (the docs should build without error).
• [n/a] New plotting related features are documented with examples.

Release Notes

• [n/a] New features are marked with a .. versionadded:: directive in the docstring and documented in doc/users/next_whats_new/
• [n/a] API changes are marked with a .. versionchanged:: directive in the docstring and documented in doc/api/next_api_changes/
• [n/a] Release notes conform with instructions in next_whats_new/README.rst or next_api_changes/README.rst