Build system: make our website cache volume smaller
Our cache garbage collection apparently keeps the actual data stored in there under 1GB, but despite this, the qcow2 file will grow up to its allocated size due to empty sectors not being garbage collected eagerly. Unfortunately, that allocated size does not fit in the host partition on our Jenkins isobuilders. We could grow those partitions, but it seems wasteful, so let's instead make the website cache volume smaller: I suppose QEMU will eventually detect the empty sectors and reclaim them, to ensure the qcow2 file does not grow over its configured maximum size.