Pull environment vars from Heroku ENV_DIR

Heroku has recently deprecated their `user-env-compile` labs feature.
Following are some notes from their email.

We are changing the way the app environment is passed to the buildpack
during builds and retiring user-env-compile. This change will simplify
how the app environment is made available during builds.

Please reach out to the maintainer of the buildpack you’re using and request
an update to use the new [API](https://devcenter.heroku.com/articles/buildpack-api-staging?preview=1#bin-compile).