Try the ppa experiment

Diff from #1119, but it's a draft PR, so I can't merge and I'm curious / eager to see all green

cc @jeroen

Author: jennybc

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

@@ -35,7 +35,7 @@ jobs:
     steps:
       - uses: actions/checkout@v2
 
-      - uses: r-lib/actions/setup-r@master
+      - uses: r-lib/actions/setup-r@ppa
         with:
           r-version: ${{ matrix.config.r }}