publish-docs "doxygen .../google/cloud/compute/Doxyfile.compute-docs" failure

* GCB: https://console.cloud.google.com/cloud-build/builds;region=us-east1/7ecda689-b751-42d0-938b-ddae4e908a77;tab=detail?project=cloud-cpp-testing-resources
* Raw: https://storage.googleapis.com/cloud-cpp-community-publiclogs/logs/google-cloud-cpp/main/0c3c395273c100ab7910063402c68b610d01a137/fedora-latest-publish-docs-publish-docs/log-7ecda689-b751-42d0-938b-ddae4e908a77.txt

```
Step #5: T+743.963s [444/511] Generate compute HTML documentation
Step #5: FAILED: google/cloud/compute/CMakeFiles/compute-docs /workspace/cmake-out/google/cloud/compute/CMakeFiles/compute-docs
Step #5: cd /workspace/google/cloud/compute && /usr/bin/cmake -E make_directory /workspace/cmake-out/google/cloud/compute && /usr/bin/doxygen /workspace/cmake-out/google/cloud/compute/Doxyfile.compute-docs
```

Unfortunately there was no further error output, and my initial investigation indicates that it is all hidden by CMake's `doxygen_add_docs()` function.

Let's at least file this issue to track if it recurs.