Update 'flake.nix'

flake.nix

@@ -406,10 +406,10 @@
                       Restart = "on-failure";
                     } // hardeningFlags;
                     preStart = ''
-                      if [ ! -d ${cfg.certDir}/${name}.${cfg.externalHost} ]; then
-                        mkdir -p ${cfg.certDir}/${name}.${cfg.externalHost}
-                      fi
                       if ${boolToString cfg.generateCerts}; then
+                        if [ ! -d ${cfg.certDir}/${name}.${cfg.externalHost} ]; then
+                          mkdir -p ${cfg.certDir}/${name}.${cfg.externalHost}
+                        fi
                         if [ ! -f ${cfg.certDir}/${name}.${cfg.externalHost}/fullchain.pem ] || \
                            [ ! -f ${cfg.certDir}/${name}.${cfg.externalHost}/privkey.pem ]; then
                           ${pkgs.libressl}/bin/openssl req -x509 -newkey rsa:4096 \