fixed typo

flake.nix

@@ -43,7 +43,7 @@
           docker_cmd="${pkgs.docker}/bin/docker run --rm -it --privileged -v $bld_dir:/vyos -w /vyos vyos/vyos-build:current"
 
           git clone -b current --single-branch https://github.com/vyos/vyos-build $root
-          $docker_cmd sudo ./build-vyos-image ${flavor} ${builtins.concatStringsSep " " bldFlags}
+          $docker_cmd sudo ./build-vyos-image ${cfg.flavor} ${builtins.concatStringsSep " " bldFlags}
           $docker_cmd sudo chown -R ${config.users.users.vyos-bld.uid}:${config.users.groups.vyos-bld.gid} /vyos
 
           cp $bld_dir/build/$iso_name ${cfg.output}