RAILS_ENV should not be set when using ps:exec

The `ps:exec` command [does not load config variables](https://devcenter.heroku.com/articles/exec#environment-variables), but does run `.profile.d` scripts. This results in incorrect values being set for `RAILS_ENV` and `RACK_ENV`.

For example, if you inspect an app's `.profile.d/ruby.sh`, you'll see:

```
~ $ cat .profile.d/ruby.sh | grep _ENV
export RAILS_ENV=${RAILS_ENV:-production}
export RACK_ENV=${RACK_ENV:-production}
```

Because your custom set `RAILS_ENV` config var is not load, the `profile.d` script sets the default value of `production`.

## Correct behavior

The `.profile.d/ruby.sh` should wrap the `export` lines in an `if` guard like:

```bash
if [[ -z "$SSH_CLIENT" ]]; then
export RAILS_ENV=${RAILS_ENV:-production}
export RACK_ENV=${RACK_ENV:-production}
fi
```

Start by tracing how the buildpack generates `.profile.d/ruby.sh` and how `ps:exec` handles environment variables. Update the generated script so the RAILS_ENV and RACK_ENV exports are guarded as shown, then verify that custom values remain available when using `ps:exec`.

Written by the indexing model from the issue text.