Adding support for environment variables in HHVM debugger task

[karoun]

[salesforce-cla[bot]]

Thanks for the contribution! Unfortunately we can't verify the commit author(s): Karoun Kasraie <k***@K***.local>. One possible solution is to add that email to your GitHub account. Alternatively you can change your commits to another email and force push the change. After getting your commits associated with your GitHub account, sign the Salesforce Inc. Contributor License Agreement and this Pull Request will be revalidated.