ci: test not capturing issue in c++ 20 only logic #15593
Description
#15563 introduced compilation error for code guarded behind a #ifdef that detects if c++20 support is available.
The presubmit running c++20 was not able to capture this compilation issue. However, internal presubmits does capture this.
https://github.com/googleapis/google-cloud-cpp/runs/51462396637
This is later fixed in #15589