Fix macOS compilation

[simon-mo]

https://buildkite.com/ray-project/ray-builders-branch/builds/1802#d42a0e6c-ff64-4c31-acb3-320d73c66ef4/629-805

src/ray/core_worker/reference_count_test.cc:280:23: error: loop variable 'callback_it' creates a copy from type 'const std::__1::pair<const ray::ObjectID, std::__1::function<void ()>>' [-Werror,-Wrange-loop-construct]
      for (const auto callback_it : callback_map) {
                      ^
src/ray/core_worker/reference_count_test.cc:280:12: note: use reference type 'const std::__1::pair<const ray::ObjectID, std::__1::function<void ()>> &' to prevent copying
      for (const auto callback_it : callback_map) {
           ^~~~~~~~~~~~~~~~~~~~~~~~
                      &
src/ray/core_worker/reference_count_test.cc:278:21: error: loop variable 'it' creates a copy from type 'const std::__1::pair<const std::__1::basic_string<char>, std::__1::unordered_map<ray::ObjectID, std::__1::function<void ()>>>' [-Werror,-Wrange-loop-construct]
    for (const auto it : subscription_failure_callback_map) {
                    ^
src/ray/core_worker/reference_count_test.cc:278:10: note: use reference type 'const std::__1::pair<const std::__1::basic_string<char>, std::__1::unordered_map<ray::ObjectID, std::__1::function<void ()>>> &' to prevent copying
    for (const auto it : subscription_failure_callback_map) {
         ^~~~~~~~~~~~~~~
                    &

This was not detected because two commits were merged right after each
other and we don't linearize merges. So the buildkite PR was green, and
the OBOD PR was green as well.

Why are these changes needed?

Related issue number

Checks

• I've run scripts/format.sh to lint the changes in this PR.
• I've included any doc changes needed for https://docs.ray.io/en/master/.
• I've made sure the tests are passing. Note that there might be a few flaky tests, see the recent failures at https://flakey-tests.ray.io/
• Testing Strategy
  • Unit tests
  • Release tests
  • This PR is not tested :(

[rkooo567]

Thanks for catching this!!

[simon-mo]

I will merge once the compile finishes

[simon-mo]

Compile passed