nix.binaryCaches: always set https://cache.nixos.org

There are many support questions when people add a new binary cache
and they suddenly lose nixos substitutions.

Most of the users want to keep that, so we're doing a breaking change.

Previously to disable all binary caches one had to do:

  nix.binaryCache = [];

Now the same is possible via:

  nix.binaryCache = lib.mkForce;