User env compile deprecated soon, keep env available for bin/compile

Heroku gave a heads up that user-env-compile will be deprecated in the following weeks. The env will be available in a directory and the directory will be given as a parameter for bin/compile.

This pull request keeps the env available after those Heroku changes. You can test this with
```
    heroku labs:enable buildpack-env-arg
    heroku labs:disable user-env-compile
```

After Heroku has made the changes enabling buildpack-env-arg is not needed, this is just for testing.