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

[AlexSkrypnyk]

#635

Update of #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.

[mglaman]

I'm curious what is the use case? There is no require-dev in the root repo? Or for allowing dev dependencies not blocked by Drupal's own requirement?

[AlexSkrypnyk]

2 use cases:

1. Older versions of Drupal core have limitations on the versions of the tooling they allow, so if someone wants to use more modern versions of tools - it is not possible to do so.

2. The website is located in a different directory than the tooling like PHPStan. This is for the cases when, for example, there are multiple websites setup for testing in sub-directories and there is a separate location for all the tooling that just runs over these different setups from a central place.