programs: nix: ship nixVersions.latest

it gives better error messages, and i'm not seeing any regressions so far

hosts/common/programs/nix.nix

@@ -1,6 +1,7 @@
-{ ... }:
+{ pkgs, ... }:
 {
   sane.programs.nix = {
+    packageUnwrapped = pkgs.nixVersions.latest;
     env.NIXPKGS_ALLOW_UNFREE = "1";  #< FUCK OFF YOU'RE SO ANNOYING
     persist.byStore.plaintext = [
       # ~/.cache/nix can become several GB; persisted to save RAM