Downgrade CMake to 3.29 to workaround Abseil issue.

This was fixed in abseil/abseil-cpp@cd7f66c, but older versions of Abseil break in CMake 3.30, which github runners now use by default.

PiperOrigin-RevId: 651854867

Author: mkruskal-google

.github/workflows/test_cpp.yml

@@ -426,6 +426,12 @@ jobs:
           arch: ${{ matrix.windows-arch || 'x64' }}
           vsversion: ${{ matrix.vsversion }}
 
+      # Workaround for Abseil incompatibility with CMake 3.30 (b/352354235).
+      - name: Downgrade CMake
+        if: ${{ runner.os == 'Windows' }}
+        run: choco install cmake --version 3.29.6 --force
+        shell: bash
+
       # Workaround for incompatibility between gcloud and windows-2019 runners.
       - name: Install Python
         if: ${{ matrix.python-version }}