[Security] Bump handlebars from 4.3.0 to 4.6.0 in /build
Bumps [handlebars](https://github.com/wycats/handlebars.js) from 4.3.0 to 4.6.0. **This update includes a security fix.**
- [Release notes](https://github.com/wycats/handlebars.js/releases)
- [Changelog](https://github.com/handlebars-lang/handlebars.js/blob/master/release-notes.md)
- [Commits](https://github.com/wycats/handlebars.js/compare/v4.3.0...v4.6.0)