[release/3.0.1xx] Update dependencies from dotnet/arcade (#4933)

* Update dependencies from https://github.com/dotnet/arcade build 20190924.3

- Microsoft.DotNet.Arcade.Sdk - 1.0.0-beta.19474.3

* Update NuGet.config

* React to NuGet design change for PackageDownload with multiple versions

A single item must be used with semicolon-delimited versions.

eng/common/sdl/extract-artifact-packages.ps1

@@ -5,6 +5,13 @@ param(
 
 $ErrorActionPreference = "Stop"
 Set-StrictMode -Version 2.0
+
+# `tools.ps1` checks $ci to perform some actions. Since the post-build
+# scripts don't necessarily execute in the same agent that run the
+# build.ps1/sh script this variable isn't automatically set.
+$ci = $true
+. $PSScriptRoot\..\tools.ps1
+
 $ExtractPackage = {
   param( 
     [string] $PackagePath                                 # Full path to a NuGet package