Update actions/checkout action to v4 (#71)

Co-authored-by: renovate[bot] <29139614+renovate[bot]@users.noreply.github.com>

.github/workflows/main.yml

@@ -21,7 +21,7 @@ jobs:
       # This is where bazelisk caches its downloads of Bazel.
       BAZELISK_HOME: /home/runner/.cache/bazel_ci/bazelisk
     steps:
-      - uses: actions/checkout@v3
+      - uses: actions/checkout@v4
       - name: Install Ubuntu dependencies
         run: .github/ci_setup.bash
       # Restore the most appropriate caches.