fix(build): ensure poetry export returns index credentials

[dotboris]

This PR fixes an issue when using stew build with a private password protected registry. When stew downloads the wheels for all dependencies, it does so by calling poetry export followed by pip wheels.

If you happen to have a primary source configured in your pyproject.toml and that this primary source is password protected, you'll get an error about pip being unable to resolve any package in your project.

This happens because by default poetry export adds an --index-url in its output but the URL it produces doesn't include credentials. This PR changes the way poetry export is called so that the --index-url now includes any credentials if they're preset.

[jonapich]

Thanks a lot!