ci: test not capturing issue in c++ 20 only logic

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https://github.com/googleapis/google-cloud-cpp/pull/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 https://github.com/googleapis/google-cloud-cpp/pull/15589