update coverage GH workflow

Author: bbolker

.github/workflows/test-coverage.yaml

@@ -30,7 +30,7 @@ jobs:
         shell: Rscript {0}
 
       - name: Restore R package cache
-        uses: actions/cache@v2
+        uses: actions/cache@v4
         with:
           path: ${{ env.R_LIBS_USER }}
           key: ${{ runner.os }}-${{ hashFiles('.github/R-version') }}-1-${{ hashFiles('.github/depends.Rds') }}