Use system method to generate temporary file path

This method is not documented, but is used by Dir.mktmpdir.  https://apidock.com/ruby/Dir/Tmpname/create

app/controllers/admin/product_import_controller.rb

@@ -128,9 +128,7 @@ module Admin
     end
 
     def mktmpdir
-      tmpdir = tmpdir_base + SecureRandom.hex(6)
-      Dir.mkdir(tmpdir)
-      tmpdir
+      Dir::Tmpname.create(TMPDIR_PREFIX, Rails.root.join('tmp') ) { |tmpname| Dir.mkdir(tmpname) }
     end
 
     def tmpdir_base