build: repair the build of the ARM64 SDK

This is needed when testing the full packaging rules.  We had
accidentally regressed this build.

Author: compnerd

build.ps1

@@ -944,6 +944,7 @@ function Install-Redist($Arch) {
   if ($ToBatch) { return }
 
   $RedistInstallRoot = Get-RuntimeInstallRoot $Arch
+  if ($null -eq $RedistInstallRoot) { return }
 
   Remove-Item -Force -Recurse $RedistInstallRoot -ErrorAction Ignore
   Copy-Directory "$($Arch.SDKInstallRoot)\usr\bin" "$RedistInstallRoot\usr"