Fix missing blobxfer on Windows on restart

- Resolves #358

scripts/windows/shipyard_nodeprep_nativedocker.ps1

@@ -82,12 +82,6 @@ if ($a) {
 	.\azurefile-mount.cmd
 }
 
-if (Test-Path $NodePrepFinished -pathType Leaf)
-{
-	Write-Host "$NodePrepFinished file exists, assuming successful completion of node prep"
-	exit 0
-}
-
 # download blobxfer binary
 $bxurl = "https://github.com/Azure/blobxfer/releases/download/${x}/blobxfer-${x}-win-amd64.exe"
 $bxoutf = Join-Path $Env:AZ_BATCH_TASK_WORKING_DIR -ChildPath "blobxfer.exe"
@@ -98,6 +92,13 @@ if (!$?)
 	throw "Download from $bxurl to $bxoutf failed"
 }
 
+# check if this script was run successfully prior
+if (Test-Path $NodePrepFinished -pathType Leaf)
+{
+	Write-Host "$NodePrepFinished file exists, assuming successful completion of node prep"
+	exit 0
+}
+
 # pull required images
 Exec { docker pull mcr.microsoft.com/azure-batch/shipyard:${v}-cargo-windows }