title: VSTS dotnet Nuget Pack: Is not a valid version string
body: <p>I am trying to create a prerelease build for a .net-standard 2.0 library in VSTS. I have created a build with the following steps</p>
  
  <ol>
  <li>dotnet restore version 2</li>
  <li>dotnet build version 2</li>
  <li>dotnet pack version 2</li>
  <li>nuget push version 2</li>
  </ol>
  
  <p>When I use the environment variable (PackageName) as <code>$(Build.BuildNumber)-beta</code> as my pack version. The pack fails with the error <code>BuildName_2018.7.11.1-beta is not a valid version string</code>. I have previously used this environment variable as my pack version in .net-framework builds with success.</p>