feat(nix): add supported search formats to Searx config

- Added `html`, `json`, and `rss` to the `search.formats` list in `searx.nix`.
- Enhances flexibility by allowing multiple output formats for search results.

systems/x86_64-linux/sgx/searx.nix

@@ -13,6 +13,11 @@
       server = {
         secret_key = config.sops.secrets."searx/secret_key".path;
       };
+      search.formats = [
+        "html"
+        "json"
+        "rss"
+      ];
     };
   };