Support Windows Powershell environment variables

[adamrtalbot]

Currently we do not support Windows Powershell variables directly. Our use of os.env means that Windows Powershell users are able to use them, but must use the syntax $env:ENV_VAR_NAME. Typically, the default Windows variables are referenced using a percentage sign, e.g. %ENV_VAR_NAME%. We should do 2 things:

• Clarify the use of Powershell environment variables in the documentation
• Support Windows env vars using the percentage sign syntax

[ejseqera]

Fixed - closing this out.