See this discussion: Best practice for build-time secrets? (spoiler, there are no answers).
I haven’t needed this, so I can’t tell if it stopped working now or whether it was never supposed to work this way (cc: @fideloper-fly), but I want to point out that you can always pass multiple --build-args to fly deploy (ref) and set them as ENVs in dockerfile which are then available as env vars to your app instance running on Fly. nb, it is not recommended to use --build-arg for sensitive data.