.github: update actions/cache

Author: bugobliterator

.github/workflows/c-cpp.yml

@@ -51,7 +51,7 @@ jobs:
         NOW=$(date -u +"%F-%T")
         echo "::set-output name=timestamp::${NOW}"
     - name: ccache cache files
-      uses: actions/cache@v2
+      uses: actions/cache@v4
       with:
         path: ~/.ccache
         key: ${{github.workflow}}-ccache-${{steps.ccache_cache_timestamp.outputs.timestamp}}