update cache to v4 (#65)

Author: wing328

.github/workflows/ci.yaml

@@ -10,7 +10,7 @@ jobs:
     steps:
       # Caches and restores the bazelisk download directory.
       - name: Cache bazelisk download
-        uses: actions/cache@v2
+        uses: actions/cache@v4
         env:
           cache-name: bazel-cache
         with: