NuGet.org had TLS 1.0 and 1.1 disabled - https://devblogs.microsoft.com/nuget/deprecating-tls-1-0-and-1-1-on-nuget-org/.

Therefore we need this line specifying Tls12

build/build-bootstrap.ps1

@@ -34,6 +34,7 @@
     if (-not (test-path $nuget))
     {
       Write-Host "Download NuGet..."
+	  [Net.ServicePointManager]::SecurityProtocol = [Net.SecurityProtocolType]::Tls12
       Invoke-WebRequest $source -OutFile $nuget
       if (-not $?) { throw "Failed to download NuGet." }
     }