feat(nix): add harmonia cache to nix.settings

Configure all NixOS machines to use the internal harmonia binary cache:
- Add nix-cache.johnogle.info as substituter
- Add harmonia signing public key to trusted-public-keys
- Enable fallback for local builds when cache unreachable
- Set 5s connect-timeout for faster fallback

Refs: x-qdkuu

Co-Authored-By: Claude Opus 4.5 <noreply@anthropic.com>

roles/common.nix

@@ -23,7 +23,13 @@
         max-jobs = "auto";
         trusted-users = [ "johno" ];
         substituters = [
+          "https://nix-cache.johnogle.info"
         ];
+        trusted-public-keys = [
+          "nix-cache.johnogle.info-1:IC5x8BxnrqkU9XqhMdDnZLtSg9Y3rBJVXhve5DJ92J0="
+        ];
+        fallback = true;
+        connect-timeout = 5;
       };
 
       gc = {