Dart CI crashes

Since a few days almost all Dart CI jobs crash. I believe this is related to either storage or memory limits (e.g. https://github.com/nextcloud/neon/actions/runs/13647643553/job/38149349606). It's probably caused by the update to 31, because we now have even more docker images. We could at least try to clean the build cache and remove any unused docker data, but as seen in the job run above it also already happens when building the docker images.