only set the OMNIBUS_DOWNLOAD_DIR env var if auto_detect is true

lib/vagrant-cachier/action/configure_bucket_root.rb

@@ -34,7 +34,7 @@ module VagrantPlugins
         def setup_omnibus_cache_folder
           # unfortunately vagrant-omnibus hooks in before our buckets are installed, yet
           # this is early enough to tell it to download to the vagrant-omnibus pseudo bucket
-          if omnibus_plugin_detected? && omnibus_plugin_enabled?
+          if env[:machine].config.cache.auto_detect && omnibus_plugin_detected? && omnibus_plugin_enabled?
             ENV['OMNIBUS_DOWNLOAD_DIR'] ||= "#{guest_cache_root}/vagrant-omnibus"
           end
         end