Sorry for not testing/reporting this earlier: the chromium snap fails to build with snapcraft 4.0.4 on the Launchpad builders.
This is because Launchpad sets up a proxy to allow the build to access the internet for a limited amount of time, and chromium takes a very long time to build (typically 8+ hours on amd64, and much longer on other architectures).
chromium’s snapcraft.yaml uses part ordering (the after keyword) to make sure that all the parts that need to fetch data/packages from the internet do so before actually building chromium. Not all the parts are specified in this ordering because not all of them actually stage packages.
But it appears that even for the parts that don’t stage packages, the new major version of snapcraft now runs apt-get update regardless, thus failing if the part is built after building chromium, because the proxy has been revoked. See this failed build for details.
I can probably work around the problem by specifying all the parts in the ordering dance, however I’d like to understand whether this new behaviour is intentional, or a regression.