nixos/gitlab: Set MALLOC_ARENA_MAX to "2"

This should reduce memory fragmentation drastically and is recommended
by both the Puma and the Sidekiq author. It's also the default value
for Ruby deployments on Heroku.

nixos/modules/services/misc/gitlab.nix

@@ -155,6 +155,7 @@ let
     GITLAB_REDIS_CONFIG_FILE = pkgs.writeText "redis.yml" (builtins.toJSON redisConfig);
     prometheus_multiproc_dir = "/run/gitlab";
     RAILS_ENV = "production";
+    MALLOC_ARENA_MAX = "2";
   };
 
   gitlab-rake = pkgs.stdenv.mkDerivation {