Add support for config vars in pip requirements files

Unsure if I'm somehow doing something wrong or this is by design. According to Pip's documentation, I can include a private repo in my requirements.txt as a dependency. Since the repo is private and hosted on GitHub, I need to provide a personal access token. Of course, I do not want that token included in source code, so I configure it in Heroku's Config Vars. But it doesn't seem to work for me. However works fine when running locally.

For example, requirements.txt:
```
aiohttp
cryptography
cachetools
sqlalchemy==1.4.46
git+https://${GITHUB_TOKEN}@github.com/user/repo.git@main
```

This is the error I get from Heroku. It appears, it does not resolve the GITHUB_TOKEN environmental variable that is defined in the Config Vars.
```
-----> Installing requirements with pip
Collecting git+https://****@github.com/user/repo.git@main (from -r requirements.txt (line 9))
Cloning https://****@github.com/user/repo.git (to revision main) to /tmp/pip-req-build-imw3lwma
Running command git clone --filter=blob:none --quiet 'https://****@github.com/user/repo.git' /tmp/pip-req-build-imw3lwma
fatal: could not read Password for 'https://${GITHUB_TOKEN}@github.com': No such device or address
error: subprocess-exited-with-error
```

Is there a workaround or something I'm missing? Thanks

https://pip.pypa.io/en/stable/reference/requirements-file-format/#using-environment-variables