[#635] Added support for env variables for Drupal root.

[AlexSkrypnyk]

#635

Update of https://github.com/mglaman/phpstan-drupal/pull/702

This helps in cases when PHPStan is called outside of the main website location.

This is still relevant in cases when tooling is outside of Drupal install.

Feel free to close, but it may help someone as well.