There doesn't seem to be an easy way to import existing corpora

I'm not sure it can be fixed easily in the sense that it seems to be hard to upload artifacts using `curl` or something like that but having spent about an hour trying to import a corpus I think the easiest way is to open a PR where the corpus is put in `$OUT/fuzz-target_seed_corpus.zip` and temporarily unblock `cflite_batch` to run it on PRs. This way, the corpus is uploaded and can be used by PRs before `cflite_batch` itself is merged.