revert(pypi): revert the default for pipstar

[aignas]

A user mentioned that flipping the pipstar default caused
a regression in their setup in this comment. Until we have
a better understanding and a regression test for that, we should
revert the flip. Adjusted the CHANGELOG notes for this.

Related to #2949

[gemini-code-assist]

Summary of Changes

Hello @aignas, I'm Gemini Code Assist¹! I'm currently reviewing this pull request and will post my feedback shortly. In the meantime, here's a summary to help you and other reviewers quickly get up to speed!

This pull request reverts a previous change, setting the pipstar feature to be disabled by default instead of enabled. This ensures that users must explicitly opt-in to use pipstar by setting the RULES_PYTHON_ENABLE_PIPSTAR environment variable, providing more control over its adoption and aligning with the intent of issue #2949.

Highlights

• Revert pipstar Default: The default value for the pipstar flag has been reverted from enabled ("1") to disabled ("0"), meaning pipstar is no longer enabled by default.
• Changelog Update: The CHANGELOG.md entry for the pipstar flag has been updated to accurately reflect its new default state and clarify how to enable it using the RULES_PYTHON_ENABLE_PIPSTAR environment variable.

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[gemini-code-assist]

Code Review

This pull request reverts the default setting for pipstar, disabling it by default. The code change in python/private/internal_config_repo.bzl is correct. However, the accompanying update to CHANGELOG.md is confusing and contains an error, stating that pipstar can be enabled with RULES_PYTHON_ENABLE_PIPSTAR=0, which is incorrect. I've provided a suggestion to clarify the changelog entry and correct the environment variable value to 1 for enabling the feature.

[aignas]

Ah, I'll need to do some extra fixes for unit tests.