Need setup-r step for lint-md (#6724)

Author: MichaelChirico

.github/workflows/code-quality.yaml

@@ -70,6 +70,7 @@ jobs:
     runs-on: ubuntu-latest
     steps:
       - uses: actions/checkout@v4
+      - uses: r-lib/actions/setup-r@v2
       - name: Lint
         run: for (f in list.files('.ci/linters/md', full.names=TRUE)) source(f)
         shell: Rscript {0}