cross-ci/lib/var/BUILD_NUM.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');

/// CI service build number.
const BUILD_NUM = () =>
    process.env.BUILD_NUM
    || process.env.CIRCLE_BUILD_NUM // CircleCI
    || process.env.TRAVIS_BUILD_NUMBER // Travis
    || process.env.CI_JOB_ID // Gitlab
    || process.env.BUILD_NUMBER // TeamCity convention
    || envCi.build
    || 0;

module.exports = BUILD_NUM;