Set the flag in the right place (#13353)

eng/Build.ps1

@@ -425,6 +425,8 @@ try {
 
     [System.Environment]::SetEnvironmentVariable('DOTNET_ROLL_FORWARD_TO_PRERELEASE', '1', [System.EnvironmentVariableTarget]::User)
 
+    $env:NativeToolsOnMachine = $true
+
     Process-Arguments
 
     . (Join-Path $PSScriptRoot "build-utils.ps1")
@@ -454,8 +456,6 @@ try {
     $env:DOTNET_ROOT = "$dotnetPath"
     Get-Item -Path Env:
 
-    $env:NativeToolsOnMachine = $true
-
     if ($bootstrap) {
         $script:BuildMessage = "Failure building bootstrap compiler"
         $bootstrapDir = Make-BootstrapBuild