Developer build does not work on MS Windows

> extracted from #782

## Problem

Docker-based release builds runn in linux container, but regular `yarn dev-build` does not, and `$RELEASE_CHANNEL` variable is not set correctly on Windows.

## Fix

Just avoid shell scripting, move to JS. In this case:

- [ ] refactor `package.json`, simplify the way vendor-specific bundles are generated.
- [ ] Create JS script that reads ENV variable and takes care of both manifest update and merging
- [ ] Remove `ci/update-manifest.sh`
- [ ] Remove manifest merging in CLI
- [ ] Ensure manifest gets updated during `bundle:` tasks