nginxMainline: use openssl_4 to support ECH

ECH is only supported in OpenSSL >=4.0,
https://openssl-library.org/post/2026-03-11-ech/index.html

We already use openssl_4 for nginxStable

pkgs/top-level/all-packages.nix

@@ -7297,6 +7297,7 @@ with pkgs;
   };
 
   nginxMainline = callPackage ../servers/http/nginx/mainline.nix {
+    openssl = openssl_4_0;
     zlib-ng = zlib-ng.override { withZlibCompat = true; };
     withKTLS = true;
     withPerl = false;