github: switch all actions to v2

* master is no more
* not sure if this will actually pass ...

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

@@ -35,11 +35,11 @@ jobs:
     steps:
       - uses: actions/checkout@v2
 
-      - uses: r-lib/actions/setup-r@master
+      - uses: r-lib/actions/setup-r@v2
         with:
           r-version: ${{ matrix.config.r }}
 
-      - uses: r-lib/actions/setup-pandoc@master
+      - uses: r-lib/actions/setup-pandoc@v2
 
       - name: Query dependencies
         run: Rscript -e "install.packages('remotes')" -e "saveRDS(remotes::dev_package_deps(dependencies = TRUE), 'depends.Rds', version = 2)"
@@ -80,7 +80,7 @@ jobs:
 
       - name: Upload check results
         if: failure()
-        uses: actions/upload-artifact@master
+        uses: actions/upload-artifact@v2
         with:
           name: ${{ runner.os }}-r${{ matrix.config.r }}-results
           path: check