github actions: upgrade to cache v4

Author: pcj

.github/workflows/ci.yaml

@@ -23,14 +23,14 @@ jobs:
       - uses: bazelbuild/setup-bazelisk@v2
 
       - name: Mount bazel action cache
-        uses: actions/cache@v3
+        uses: actions/cache@v4
         if: always()
         with:
           path: "~/.cache/bazel"
           key: bazel
 
       - name: Mount bazel repo cache
-        uses: actions/cache@v3
+        uses: actions/cache@v4
         if: always()
         with:
           path: "~/.cache/bazel-repo"