[ci] [R-package] pin to older setup-pandoc for R 3.6 jobs (fixes #6298) (#6299)

.github/workflows/r_package.yml

@@ -179,6 +179,13 @@ jobs:
           submodules: true
       - name: Install pandoc
         uses: r-lib/actions/setup-pandoc@v2
+        if: matrix.container != 'ubuntu:18.04'
+      # R 3.6 binary isn't easily available on buntu 18.04,
+      # but setup-pandoc>=2.7.1 is uses a too-new glibc for it.
+      # ref: https://github.com/microsoft/LightGBM/issues/6298
+      - name: Install pandoc
+        uses: r-lib/actions/setup-pandoc@v2.6.0
+        if: matrix.container == 'ubuntu:18.04'
       - name: install tinytex
         if: startsWith(matrix.os, 'windows')
         uses: r-lib/actions/setup-tinytex@v2