Fix github action r tests. (dmlc#8364)

Author: trivialfis

.github/workflows/r_tests.yml

@@ -26,7 +26,7 @@ jobs:
       with:
         submodules: 'true'
 
-    - uses: r-lib/actions/setup-r@master
+    - uses: r-lib/actions/setup-r@v2
       with:
         r-version: ${{ matrix.config.r }}
 
@@ -75,7 +75,7 @@ jobs:
       with:
         submodules: 'true'
 
-    - uses: r-lib/actions/setup-r@master
+    - uses: r-lib/actions/setup-r@v2
       with:
         r-version: ${{ matrix.config.r }}
 
@@ -126,11 +126,11 @@ jobs:
       with:
         submodules: 'true'
 
-    - 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-tinytex@master
+    - uses: r-lib/actions/setup-tinytex@v2
 
     - name: Install system packages
       run: |