add NIX_CONF_DIR to sudo env_keep variables (suggested by Eelco Dolstra)

this enables nix-collect-garbage under sudo to respect nix.conf, e.g.:

    gc-keep-outputs = true
    gc-keep-derivations = true

modules/security/sudo.nix

@@ -34,6 +34,7 @@ in
 
           # Environment variables to keep for root and %wheel.
           Defaults:root,%wheel env_keep+=LOCALE_ARCHIVE
+          Defaults:root,%wheel env_keep+=NIX_CONF_DIR
           Defaults:root,%wheel env_keep+=NIX_PATH
           Defaults:root,%wheel env_keep+=TERMINFO_DIRS