Add a host for testing i_only_boot_zfs.

nix/configuration/configuration.nix

@@ -23,6 +23,7 @@
     ./roles/distributed_build
     ./roles/doas
     ./roles/docker
+    ./roles/dont_use_substituters
     ./roles/ecc
     ./roles/emacs
     ./roles/emulate_isa
@@ -105,13 +106,10 @@
     hardware.enableRedistributableFirmware = true;
 
     # Keep outputs so we can build offline.
-    # Disable substituters to avoid risk of cache poisoning.
     nix.extraOptions = ''
       keep-outputs = true
       keep-derivations = true
-      substitute = false
     '';
-    nix.settings.substituters = lib.mkForce [ ];
 
     # Automatic garbage collection
     nix.gc = lib.mkIf (!config.me.buildingPortable) {