AzurePipelinesCredential: clarify system access token requirement

[chlowell]

Currently the credential requires users to provide the system access token via the environment variable SYSTEM_ACCESSTOKEN. Users must configure their pipelines to set this variable because AzDO doesn't do so automatically. We must at least document this requirement. We should also consider taking the system access token as a constructor argument rather than an environment variable. Doing that would make the value obviously required and make the credential's configuration API consistent in that every configuration parameter required from the application would be a constructor argument.

[AisseMTT]

Can we also add documentation for how to set the SYSTEM_ACCESSTOKEN in AzDO? @chlowell perhaps you have some documentation for how to do this?

[chlowell]

👍 there's a YAML example in the Pipelines documentation