cross-ci/lib/var/GITHUB_TOKEN.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.

/// Equals to `GITHUB_TOKEN`, `GITHUB_ACCESS_TOKEN`, `GH_TOKEN`, or `GIT_CREDENTIALS` environment variables, in that order.
const GITHUB_TOKEN = () =>
    process.env.GITHUB_TOKEN
    || process.env.GITHUB_ACCESS_TOKEN
    || process.env.GH_TOKEN
    || process.env.GIT_CREDENTIALS

    // Falsy by default, because scripts should throw if GITHUB_TOKEN not provided.
    || '';

module.exports = GITHUB_TOKEN;