Update sandpaper-main.yaml

Change install-r to true to see if fixes build issue as per carpentries/sandpaper#605

Author: lauracmurphy

.github/workflows/sandpaper-main.yaml

@@ -38,7 +38,7 @@ jobs:
         uses: r-lib/actions/setup-r@v2
         with:
           use-public-rspm: true
-          install-r: false
+          install-r: true
 
       - name: "Set up Pandoc"
         uses: r-lib/actions/setup-pandoc@v2