Needs an export file for running on CI

When running on CI I get this:

```
-----> heroku-cli app detected
=== Fetching and vendoring Heroku CLI into slug
=== Installing profile.d script
heroku/7.22.7 linux-x64 node-v11.10.1
=== Heroku CLI installation done
-----> Ruby app detected
bash: /tmp/buildpacks/be6d1a8141ea11e9dcd5364e31ecb61cbd57dcbaaab2823a85be72aa27407c6ca545b19fc22fd2ec0a4a456498d20fbcb40c682ee74dc5019358a712a0dfde5e/export: No such file or directory
```

The cause is likely the same as https://github.com/heroku/heroku-buildpack-ruby/pull/858, that not export file is being written.

Start at the buildpack entry point responsible for the export file and compare the behavior with heroku-buildpack-ruby PR #858. Done means the export file is written during CI builds and the reported missing-file error no longer occurs.

Written by the indexing model from the issue text.