Make extension work on MacOS/Linux

[MarcBruins]

NuKeeper now works on mono so the extension should too. The extension will break currently if you run it on non windows with a private feed. This happens because only windows supports encrypted nuget.config passwords, on linux/mac this is not supported.

To make this work we should change the password to be a ClearTextPassword, we delete the nuget.config in a postdeployment job and we should be able to target non windows agents