Hide Persistent Storage onboarding when not running from a USB stick

I just noticed that the work from #15586 (closed) is also display when running from an ISO image, saying "You can save" when it's actually not possible.

I'm sorry if this wasn't clear in #15586 (closed) but we should only display this Persistent Storage onboarding when it's actually possible.

I understand that it might be hard to predict from the Welcome Screen if it's 100% possible to create a Persistent Storage, but maybe we can at least check that we're running from a USB stick and not from an ISO image?