ci/eval/diff: avoid copying paths.json

Currently the `diff-<system>` artifacts are 6-7 MB in size - and almost
all of that is the `paths.json` file, which is only used to generate the
diff itself. This had been stored in the artifact previously for
debugging purposes. Ever since we moved to Cachix this is not required
anymore, since it's possible to run the same eval locally and thus fetch
the `eval.singleSystem` result, including `paths.json`, from Cachix.

This will be even more helpful when the next step adds `meta.json` -
which is magnitudes bigger than `paths.json`.

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