fix bug during download nuget (#1881)
fix bug during download nuget (#1881)

* fix bug during download nuget

* set $lastexitcode to null in begining

hellosnow/docfx,pascalberger/docfx,pascalberger/docfx,dotnet/docfx,pascalberger/docfx,superyyrrzz/docfx,DuncanmaMSFT/docfx,superyyrrzz/docfx,dotnet/docfx,superyyrrzz/docfx,hellosnow/docfx,dotnet/docfx,hellosnow/docfx,928PJY/docfx,DuncanmaMSFT/docfx,928PJY/docfx,928PJY/docfx