Update dependencies from https://github.com/dotnet/arcade build 20200129.8 (#6261)

- Microsoft.DotNet.Arcade.Sdk - 5.0.0-beta.20079.8

eng/common/generate-graph-files.ps1

@@ -3,7 +3,7 @@ Param(
   [Parameter(Mandatory=$true)][string] $gitHubPat,      # GitHub personal access token from https://github.com/settings/tokens (no auth scopes needed)
   [Parameter(Mandatory=$true)][string] $azdoPat,        # Azure Dev Ops tokens from https://dev.azure.com/dnceng/_details/security/tokens (code read scope needed)
   [Parameter(Mandatory=$true)][string] $outputFolder,   # Where the graphviz.txt file will be created
-  [string] $darcVersion = '1.1.0-beta.19175.6',         # darc's version
+  [string] $darcVersion,                                # darc's version
   [string] $graphvizVersion = '2.38',                   # GraphViz version
   [switch] $includeToolset                              # Whether the graph should include toolset dependencies or not. i.e. arcade, optimization. For more about
                                                         # toolset dependencies see https://github.com/dotnet/arcade/blob/master/Documentation/Darc.md#toolset-vs-product-dependencies