Updated actions/checkout to v3 in GitHub Actions CI workflow. (#360)

.github/workflows/c-cpp.yml

@@ -65,7 +65,7 @@ jobs:
           sudo apt-add-repository ${{ matrix.config.toolchain }}
           sudo apt-get update
           sudo apt-get -y install libtool
-    - uses: actions/checkout@v2
+    - uses: actions/checkout@v3
     - name: tweak multi_client_test.cc for container usage
       if: matrix.config.container == 'ubuntu:14.04'
       run: |