Installation fails on Windows Server Core

[clawrenceks]

I have recently been using the acr-docker-credential-helper and have found that out of the box this fails on Windows Server Core. In my testing this seems only to be because the use of Invoke-WebRequest in both the documentation and install script does not include -UseBasicParsing

When I add this additional option I am successful in my use of the azr-docker-credential-helper.

I am happy to make the change and submit a pull request if you like.