--system-site-packages should be customizable by virtualenv_create

[jiasli]

Provide a detailed description of the proposed feature

#11611 specifies --system-site-packages when creating a virtual environment. This makes Azure CLI installed by Homebrew conflict with Ansible collection for Azure installed to system site-packages using pip. (Azure/azure-cli#19027, Homebrew/discussions#2009).

This makes Homebrew less flexible, even though it fixes #1790.

What is the motivation for the feature?

To makes Homebrew more flexible.

How will the feature be relevant to at least 90% of Homebrew users?

No

What alternatives to the feature have been considered?

Let the users create virtual env by themselves when installing with pip.

[carlocab]

I had come to the same conclusion after our discussion at Homebrew/discussions#2009. This would be good to have, and should be straightforward to implement.