envoy: Envoy failed on Bazel CI with "unused variable 'factory_name'"

https://buildkite.com/bazel/envoy/builds/1295#b92e2abd-a008-49b7-a929-2a5dde5a0152

bazel-out/k8-fastbuild/bin/include/envoy/registry/_virtual_includes/registry/envoy/registry/registry.h:348:44: error: unused variable 'factory_name' [-Werror=unused-variable]
     for (const auto& [factory_name, factory] : factories()) {
                                            ^
cc1plus: all warnings being treated as errors