Support using env vars in pip `requirements.txt` files (for GitHub PATs for private repositories)

### Required Terms

- [X] I agree to follow this project's [Code of Conduct](https://github.com/heroku/roadmap/blob/main/CODE_OF_CONDUCT.md)
- [X] I have read and accept the [Salesforce Program Agreement](https://www.salesforce.com/company/program-agreement/)

### What service(s) is this request for?

_No response_

### Tell us about what you're trying to solve. What challenges are you facing?

Heroku doesn't seem to support personal access tokens for private GitHub repositories in `requirements.txt` for Python. I tried setting my GitHub PAT as a config var but got hit with "fatal: could not read Password for 'https://${GITHUB_PAT}@github.com': No such device or address" in the build log.

In the [docs](https://devcenter.heroku.com/articles/python-pip), Heroku suggests using HTTP Basic Authentication through the user:password format. However, this method [has been deprecated on GitHub since 2021](https://github.blog/changelog/2021-08-12-git-password-authentication-is-shutting-down/).

Is there any reason why Heroku can't do this natively? Any help would be appreciated.