Avoid specifying the full nugetConfigPath

Even though this property was copied verbatim from the documentation for
the NuGet task, it doesn't work.

https://docs.microsoft.com/en-us/azure/devops/pipelines/tasks/package/nuget?view=azure-devops

Author: sharwell

azure-pipelines.yml

@@ -28,7 +28,7 @@ jobs:
     inputs:
       restoreSolution: '$(BuildSolution)'
       feedsToUse: 'config'
-      nugetConfigPath: '$(Build.WorkingDirectory)/NuGet.config'
+      nugetConfigPath: 'NuGet.config'
 
   - task: VSBuild@1
     displayName: 'Build solution StyleCopAnalyzers.sln'