Builder instances are not reusable

[w1ndy]

If I create a builder instance outside a task and use the builder to bundle a directory, the bundled file is not updated as source code changes.

var builder = systemjsBuilder('./', './system.config.js');
gulp.task('bundle', () => {
    return builder
        .bundle('[./dist/**/*.js]')
        .pipe(plugins.rename('bundle.js'))
        .pipe(gulp.dest('./'))
});

Creating a builder every time the task is executed resolves the problem. Is there any way to make the builder reusable across tasks? Seems pretty costly to read the configuration file frequently.