Update dependencies from https://github.com/dotnet/arcade build 20190501.6 (#1827)

- Microsoft.DotNet.Arcade.Sdk - 1.0.0-beta.19251.6

eng/common/init-tools-native.ps1

@@ -41,9 +41,13 @@ Param (
   [switch] $Force = $False,
   [int] $DownloadRetries = 5,
   [int] $RetryWaitTimeInSeconds = 30,
-  [string] $GlobalJsonFile = "$PSScriptRoot\..\..\global.json"
+  [string] $GlobalJsonFile
 )
 
+if (!$GlobalJsonFile) {
+  $GlobalJsonFile = Join-Path (Get-Item $PSScriptRoot).Parent.Parent.FullName "global.json"
+}
+
 Set-StrictMode -version 2.0
 $ErrorActionPreference="Stop"