R-CMD-check.yaml: Use github/checkout v4

Author: kuadrat

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

@@ -30,7 +30,7 @@ jobs:
       R_KEEP_PKG_SOURCE: yes
 
     steps:
-      - uses: actions/checkout@v3
+      - uses: actions/checkout@v4
 
       - uses: r-lib/actions/setup-pandoc@v2