update cache to avoid GHA error

Author: njtierney

.github/workflows/R-CMD-check.yaml

@@ -56,7 +56,7 @@ jobs:
           echo "::set-output name=date::$(date -u "+%F")"
 
       - name: Restore R package cache
-        uses: actions/cache@v2
+        uses: actions/cache@v4
         id: r-package-cache
         with:
           path: ${{ env.R_LIBS_USER }}