[01] Eval/IO overhead in the greedy demo

Running `evanix` on https://github.com/ggerganov/llama.cpp takes, in the `--dry-run` mode, about 20s, whereas running `nix-eval-jobs` takes about 6s. Per @sinanmohd's hypothesis, this is due to querying the substituters about the cached store paths too much (does nix not maintain a negative response cache for these queries?). The performance needs to be brought down to the same order of magnitude as `nix-eval-jobs` so that we can stop talking about the greedy PoC