document EAS_PROJECT_ROOT for no VCS workflow

[trajano]

EAS_PROJECT_ROOT does not appear to be documented but it is useful for No VCS with mono repo.

An example of using this https://github.com/trajano/expo-experiments/blob/b0c536f9403ed3105bcae276afeb05ba9e7e5a1e/Dockerfile#L87

The code it documents is

https://github.com/expo/eas-cli/blob/24245d76cf6120ed9997fc2c2cdce82d605c93de/packages/eas-cli/src/vcs/local.ts#L14-L20

There was an article about it though not in a monorepo context https://dev.to/louisgv/undocumented-eas-flag-in-expo-sdk-43-4d7h

[trajano]

This ideally should link to https://github.com/expo/fyi/blob/main/eas-build-archive.md#how-do-i-opt-out-of-using-git but not sure how to set up that link on the MD.