zipdetails --walk changes trigger seemingly unrelated failures in zipdetails version check

As of 661adfc0 diffoscope can no longer build with an older zipdetails.

I presume @lamby already noticed, as stable-bpo checks were disabled in 8425d51c

This also causes failures on guix:

It seems to be failing even on tests that do not have a versioned zipdetails requirement, probably because it is calling --walk regardless of the version... so the change to use --walk may need a fallback if using an older zipdetails version... dropping zip support entirely just because of an old version would be a bit unfortunate.

To upload designs, you'll need to enable LFS and have an admin enable hashed storage. More information