Consider pruning osbuild data from cache qcow2

[jlebon]

Once in a while, when building a disk image I hit ENOSPC on the cache qcow2 so then I have to delete it so it gets recreated. I think the issue here is that we never prune from there. But it would make sense to do so whenever we prune builds.

[dustymabe]

hmm. we try to limit the amount of data used by telling OSBuild a size for its cache:

https://github.com/coreos/coreos-assembler/blob/dcce259d79d37478a91b3e2c2919dc3103c55e65/src/runvm-osbuild#L134

If the GC there wasn't working you wouldn't be able to do more than a few builds without it filling up.

It would be nice if we characterized the leak a bit more to find exactly what directories were only getting larger over time.