Investigate and fix issue with unexpected build_id changes

Example, building 16.0a1 at tbb-16.0a1-build1:

> rbm/rbm showconf firefox filename --target torbrowser-macos-aarch64 --target alpha
firefox-tor-browser-1b34bbe425f6-macos-aarch64-bdcf24
> rbm/rbm showconf browser build  --target alpha --target torbrowser-macos
...
tar -C $ff_aarch64 -xf firefox-tor-browser-1b34bbe425f6-macos-aarch64-2aafa2/browser.tar.zst
...

So, there's an unexpected change of build id.