ci: make R not old and install faster

Author: machow

.github/workflows/ci.yml

@@ -85,7 +85,7 @@ jobs:
 
     - uses: r-lib/actions/setup-r@v2
       with:
-        r-version: '3.5.3'
+        use-public-rspm: true
 
     - name: Install R dependencies
       run: "install.packages('pins')"