How can I set an environment variable as gulp task?

gulp.task('set-dev-node-env', function() {
    return process.env.NODE_ENV = 'development';
});

gulp.task('set-prod-node-env', function() {
    return process.env.NODE_ENV = 'production';
});

Use it like:

gulp.task('build_for_prod', ['set-prod-node-env'], function() {
    // maybe here manipulate config object  
    config.paths.src.scripts = config.paths.deploy.scripts;
    runSequence(
        'build',
        's3'
    );
});