Add missing script (#5622)

Author: wiktork

eng/release/Scripts/SetTaskVariable.ps1

@@ -0,0 +1,10 @@
+param(
+    [Parameter(Mandatory=$true)][string] $Name,
+    [Parameter(Mandatory=$false)][string] $Value
+)
+
+$ErrorActionPreference = 'Stop'
+$VerbosePreference = 'Continue'
+Set-StrictMode -Version 2.0
+
+Write-Host "##vso[task.setvariable variable=$Name]$Value"