Merge pull request #26843 from dotnet/marcpopMSFT-fixpackageinstallertests

Author: marcpopMSFT

build/RunTestsOnHelix.cmd

@@ -24,5 +24,5 @@ dotnet new
 REM avoid potetial cocurrency issues when nuget is creating nuget.config
 dotnet nuget list source
 REM We downloaded a special zip of files to the .nuget folder so add that as a source
-dotnet nuget add source %DOTNET_ROOT%\.nuget
-dir /B %DOTNET_ROOT%\.nuget
+dotnet new nugetconfig
+dotnet nuget add source %DOTNET_ROOT%\.nuget --configfile nuget.config