pipenv: Injecting credentials broken

Issue description

Upgrading to the latest version of pipenv via HomeBrew breaks injecting credentials using environment variables.

Expected result

pipenv install installs dependencies from private repositories.

Actual result

[pipenv.exceptions.ResolutionFailure]: Warning: Your dependencies could not be resolved. You likely have a mismatch in your sub-dependencies.
  You can use $ pipenv run pip install <requirement_name> to bypass this mechanism, then run $ pipenv graph to inspect the versions actually installed in the virtualenv.
  Hint: try $ pipenv lock --pre if it is a pre-release dependency.

Steps to replicate

pipenv install