Make sure we prune the caches after each stage-test to keep our disk
footprint small. This does considerably increase build-times since we
no longer share cached entries. However, the current CI builds simply
run out of disk-space.
Once we use separate output-directories we will be able to drop the
automatic checkpointing from the tests, and thus effectively get the
same behavior. Until then, lets prune the caches explicitly.
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