add a script to check nuget config for PRs (#2042)

build/scripts/VerifyNugetConfig.ps1

@@ -0,0 +1,28 @@
+# Copyright (c) Microsoft Corporation. All rights reserved.
+# Licensed under the MIT License.
+
+<#
+.SYNOPSIS
+  Verify the specified nuget.config. Throw to fail the pipeline if the config is ill-formed.
+.PARAMETER FilePath
+  The path pointing to the nuget.config file to be verified by this script.
+.EXAMPLE
+  VerifyNugetConfig -FilePath .\nuget.config
+#>
+
+param([Parameter(Mandatory)][string]$FilePath)
+
+$doc = [XML](Get-Content $FilePath)
+$sources = (Select-Xml $doc -XPath "//configuration/packageSources").Node
+$count = 0
+foreach($src in $sources.ChildNodes){
+  if ($src.Name -eq "add"){
+    ++$count
+  }
+  elseif ($src.Name -eq "clear"){
+    $count = 0
+  }
+}
+if ($count -gt 1){
+  throw "Adding multiple package sources is not allowed in nuget.config."
+}