add rocker/r2u container for updated validate-ctv GHA with improved performance

Author: zeileis

.github/workflows/validate-ctv.yml

@@ -19,5 +19,7 @@ name: Validate task view
 jobs:
   validate-ctv:
     runs-on: ubuntu-latest
+    container:
+      image: rocker/r2u:latest
     steps:
-      - uses: cran-task-views/ctv/validate-ctv@main
+      - uses: cran-task-views/ctv/validate-ctv@main