Build components for which dependencies are already available
We are downloading the SHA5125UMS file right in the beginning of the Tor Browser build but actually need that file only at the end for verifying the lang packs. Arthur had the idea to download it at the end as well. The rationale is that we can save time building because the lang packs is the last thing that shows up on Mozilla's servers while the build tag is way earlier available that would allow us to deliver an update faster and/or have more time for QA.
The general idea Arthur had is building intelligently only those components that can already be built and once there are none left but not everything got build we should fail.
Not sure whether that is worth the effort, though, given that it in practice should only affect the Mozilla lang pack situation we landed in right now (and previously a bunch of times)