Add a host for testing i_only_boot_zfs.

nix/configuration/roles/dont_use_substituters/default.nix

@@ -0,0 +1,28 @@
+{
+  config,
+  lib,
+  pkgs,
+  ...
+}:
+
+{
+  imports = [ ];
+
+  options.me = {
+    dont_use_substituters.enable = lib.mkOption {
+      type = lib.types.bool;
+      default = false;
+      example = true;
+      description = "Whether we want to install dont_use_substituters.";
+    };
+  };
+
+  config = lib.mkIf config.me.dont_use_substituters.enable {
+    # Disable substituters to avoid risk of cache poisoning.
+    nix.extraOptions = ''
+      substitute = false
+    '';
+
+    nix.settings.substituters = lib.mkForce [ ];
+  };
+}