Seed-gen should copy seeds only to local corpus

[reytchison]

Seed-gen currently copies seeds to the node-local corpus and remote corpus: https://github.com/trailofbits/buttercup/blob/993fa2241af5eb9b8532b9d5f36b942c5668f4ba/seed-gen/src/buttercup/seed_gen/seed_gen_bot.py#L242 This bypasses the merger-bot, which means that all seed-gen seeds are copied, instead of only the ones which add coverage. Seed-gen should instead only copy to the local corpus and let the merger-bot copy seeds to the remote corpus.