Disable the nix binary cache.

It is technically a risk and since I build most of my software anyway, I'm not getting much benefit.

nix/configuration/configuration.nix

@@ -100,6 +100,7 @@
   nix.extraOptions = ''
     keep-outputs = true
     keep-derivations = true
+    substitute = false
   '';
 
   # Technically only needed when building the ISO because nix detects ZFS in the filesystem list normally. I basically always want this so I'm just setting it to always be on.
@@ -140,6 +141,7 @@
     options = "--delete-older-than 30d";
   };
   nix.settings.auto-optimise-store = !config.me.buildingIso;
+  nix.settings.substituters = lib.mkForce [ ];
 
   # Use doas instead of sudo
   security.doas.enable = true;