Use PowerShell 5 compatible argument handling in scratch runner

scripts/run-clean-scratch-iterations.ps1

@@ -95,13 +95,11 @@ function Invoke-External {
 
         $startInfo = [System.Diagnostics.ProcessStartInfo]::new()
         $startInfo.FileName = $startFilePath
+        $startInfo.Arguments = (($startArgumentList | ForEach-Object { Format-ProcessArgument -Value ([string]$_) }) -join ' ')
         $startInfo.WorkingDirectory = $WorkingDirectory
         $startInfo.UseShellExecute = $false
         $startInfo.RedirectStandardOutput = $true
         $startInfo.RedirectStandardError = $true
-        foreach ($argument in $startArgumentList) {
-            [void]$startInfo.ArgumentList.Add([string]$argument)
-        }
 
         $process = [System.Diagnostics.Process]::new()
         $process.StartInfo = $startInfo