fix: workaround CMake file stomping (#4823)
fix: workaround CMake file stomping (#4823)

Without these changes, and only on my large VM, CMake would generate an
invalid `google_cloud_cpp_common.bzl` file. It seems that the `APPEND`
and `WRITE` steps for the file were running in parallel. I do not
understand the problem well, or why this solution helps. I can only
speculate that maybe Ninja aggressively parallelizes the file generation
steps unless they are part of the same CMake function.
googleapis/google-cloud-cpp,googleapis/google-cloud-cpp,googleapis/google-cloud-cpp,googleapis/google-cloud-cpp