configs.nix-farm: don't push paths cache.nixos.org serves
Builders pushed every path of a closure, including everything they had
substituted from cache.nixos.org: 9.9 of 10.3 GiB for ignavia. niks3 now
comes from a fork (lassulus/niks3) with `push --ignore-upstream-key`,
and neoprism's daemon and the CI farm pass cache.nixos.org-1 to it.
cache.lassul.us no longer holds complete closures on its own: clients
need cache.nixos.org next to it, which every machine has.