Update actions/cache@v2, which is soon to be retired by Github actions (#546)

Signed-off-by: Andy Fingerhut <[email protected]>

Author: jafingerhut

.github/workflows/ci-build-proto.yml

@@ -23,7 +23,7 @@ jobs:
         submodules: recursive
 
     - name: Mount bazel cache
-      uses: actions/cache@v2
+      uses: actions/cache@v4
       with:
         # See https://docs.bazel.build/versions/master/output_directories.html
         path: "~/.cache/bazel"