Test: revert to checkout@v4

Author: TroyHernandez

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

@@ -22,7 +22,7 @@ jobs:
     runs-on: ${{ matrix.os }}
 
     steps:
-      - uses: actions/checkout@v6
+      - uses: actions/checkout@v4
 
       - name: Setup
         uses: eddelbuettel/github-actions/r-ci@master