Use 4 threads for cmake build and test execution.

The GitHub public runners have 3-4 cores:
https://docs.github.com/en/actions/using-github-hosted-runners/about-github-hosted-runners/about-github-hosted-runners#standard-github-hosted-runners-for-public-repositories

This should significantly speed up CMake CI builds, which take 10-11 minutes
compared to 2-3 minutes for Bazel builds.

PiperOrigin-RevId: 627485464

.github/workflows/cmake-test.yaml

@@ -22,7 +22,7 @@ jobs:
       run: cmake .
 
     - name: Build
-      run: cmake --build .
+      run: cmake --build . -j4
 
     - name: Test
-      run: ctest
+      run: ctest -j4