nixos/gitlab: add proxyWebsockets as recommended nginx setting (#431884)

nixos/modules/services/misc/gitlab.md

···
     virtualHosts."git.example.com" = {
       enableACME = true;
       forceSSL = true;
-      locations."/".proxyPass = "http://unix:/run/gitlab/gitlab-workhorse.socket";
+      locations."/" = {
+        proxyPass = "http://unix:/run/gitlab/gitlab-workhorse.socket";
+        proxyWebsockets = true;
+      };
     };
   };
 }