This is a step towards T67056. We need to be able to checkout a release branch
with correct submodules and libraries. The solution is to extend "make update"
to do it automatically when on a release branch, and use that for buildbot.
- Build: move "make update" on macOS and Linux to Python script
- Build: support updating for release branches in make_update.py
- Buildbot: add script to update libraries and submodules
Beyond this it would be good to use make_update.py for the Windows make.bat.
This wasn't trivial so I left it for later work.