Not 100% sure, I need to investigate a bit further, but my educated guess is that this is stored in the etcd cluster (even maybe in the build configuration metadata).
I don't think it can be stored in the node (VM or bare metal). If the archive were stored in the node, the next incremental build would be restricted to the same node. Same build in a different node and that is indeed not the case.
Let me know if this answers your question. Else I will investigate a bit more.