cross-ci/lib/var/BUILD_DIR.js

`cross-ci` standardizes environment variables for CI. For example, your can simply use `BUILD_BRANCH` variable in all CI runners instead of `CIRCLE_BRANCH` in CircleCI or `TRAVIS_PULL_REQUEST_BRANCH` in Travis.

const envCi = require('../envCi');

/// Path to repository folder.
const BUILD_DIR = () =>
    process.env.CIRCLE_WORKING_DIRECTORY
    || process.env.TRAVIS_BUILD_DIR
    || process.env.CI_PROJECT_DIR // Gitlab
    || envCi.root
    || process.cwd();

module.exports = BUILD_DIR;