Can BfabricArchiveSpec be extended to only unarchive if necessary?

[leoschwarz]

This one is tricky.

Even when the zip file has not changed the input file might have been modified, one could work with timestamps to identify if such a case might be present.

However, CRC32 checksums could provide a very reliable way to do it. Maybe with something like https://github.com/overcat/fastcrc