Summary:
The environment changed since I tested  D20740410 and now it appears
that we'll need to re-export a versioned variable in order for cmake
to detect boost on the GH actions hosts.
Pull Request resolved: https://github.com/facebook/folly/pull/1359

Test Plan:
the GH actions status of this diff.

The workflow was updated via:

```
python3 build/fbcode_builder/getdeps.py generate-github-actions folly --output-file .github/workflows/main.yml
```

Reviewed By: yfeldblum

Differential Revision: D21307640

Pulled By: yfeldblum

fbshipit-source-id: 1555cbcade822775379cd9054be37fdbc17b4d93