venv environment picker polish

Testing #21878

Placeholder text is usually an action/directive rather than a question ([guidance](https://code.visualstudio.com/api/ux-guidelines/quick-picks)), in this case it could say "Select a venv environment option" or something like that
 
![Image](https://github.com/microsoft/vscode-python/assets/30305945/6b4c515a-62db-49eb-ae60-fe0afcbeb3a1)


