Capture distro package cache differently

`REPRO_RUN_KEY` is a blunt, if generic, instrument.

e.g `https://snapshot.debian.org/archive/debian/<SOURCE_DATE_EPOCH>/` is a thing, see https://github.com/reproducible-containers/repro-sources-list.sh

Also see https://github.com/reproducible-containers/repro-pkg-cache which could be a nice thing too.