nixos/nginx: set X-Forwarded-Server proxy header to hostname

X-Forwarded-Server represents the last server in a row of reverse proxies
in the common use, see:
- https://www.fastly.com/documentation/reference/http/http-headers/X-Forwarded-Server/
- https://httpd.apache.org/docs/2.4/mod/mod_proxy.html#x-headers
- https://docs.valsight.com/on-premise/latest/reverse-proxy

X-Forwarded-Host instead is the original request host.

This change adapts our NGINX module to the common use of this header.

nixos/modules/services/web-servers/nginx/default.nix

@@ -108,7 +108,7 @@ let
     proxy_set_header        X-Forwarded-For $proxy_add_x_forwarded_for;
     proxy_set_header        X-Forwarded-Proto $scheme;
     proxy_set_header        X-Forwarded-Host $host;
-    proxy_set_header        X-Forwarded-Server $host;
+    proxy_set_header        X-Forwarded-Server $hostname;
   '';
 
   proxyCachePathConfig = concatStringsSep "\n" (