Update build.yml

Author: Bertk

eng/build.yml

@@ -20,7 +20,10 @@ steps:
     New-Item -ItemType Directory -Path artifacts/package/release -Force
   displayName: create folder artifacts/package/$(BuildConfiguration)
 
-- script: dotnet restore --configfile nuget.config
+- task: NuGetAuthenticate@1
+  displayName: Authenticate with NuGet feeds
+
+- script: dotnet restore --configfile  $(Build.SourcesDirector)/nuget.config
   displayName: Restore packages
 
 - script: dotnet build -c $(BuildConfiguration) --no-restore -bl:build.msbuild.binlog