Make it work on modern Heroku, use heroku-buildpack-multi.

Changes:
- Use the new `ENV_DIR` system instead of `user-env-compile` which no longer exists.
- Make it work with @ddollar's `heroku-buildpack-multi` since `heroku-buildpack-compose` doesn't support `ENV_DIR`.
- Make `detect` always exit zero since we can't get environment variables at that point.
- Remove the contents of `release` since it wasn't actually setting anything.