Do not require setting proxy password in environment variable

[matosconsulting]

Description

The FAQ states to use an environment variable with a password in order to authenticate to a proxy. This exposes the user's password to any application with access to the environment variables.

Expected behavior

Although I strongly disagree with the approach of using an environment variable for this purpose, I understand for some use cases this is valid. For users who would prefer to provide credentials at run/build time, please prompt the user for the credentials if they were not provided in the environment variable.