nginx: compress nix archives with zstd

modules/nginx.nix

@@ -70,6 +70,13 @@ in
           };
         }
 
+        (lib.mkIf cfg.recommendedZstdSettings {
+          commonHttpConfig = ''
+            # TODO: upstream this?
+            zstd_types application/x-nix-archive;
+          '';
+        })
+
         (lib.mkIf cfg.allRecommended (libS.modules.mkRecursiveDefault {
           recommendedBrotliSettings = true;
           recommendedGzipSettings = true;