Issue description

I am using private repo to install packages and it is refered in Pipfile.lock as like below

"sources": [
  "url":"https://$USER:$ACCESS_TOKEN@gitlab.com/private/repo"
]

Here, pipenv requirements > requirements.txt returns file using environment variables in unsupported format for pip install -r requirements.txt

Expected result(MODIFIED IN COMMENT)

# requirements.txt
-i https://expanded_username:expanded_token@gitlab.com/private/repo   
random_lib == 0.0.0  
...

pipenv lock --requirements > requirements.txt returned expected result(this command no longer available)

Actual result

# requirements.txt
-i https://$USER:$ACCESS_TOKEN@gitlab.com/private/repo   
random_lib == 0.0.0  
...

Only POSIX format is supported for pip install -r.
https://pip.pypa.io/en/stable/reference/requirements-file-format/#using-environment-variables