Install dev rmarkdown on oldrel-4

Author: krlmlr

.github/workflows/install/action.yml

@@ -77,3 +77,12 @@ runs:
         sudo ln -s $(which true) /usr/local/bin/qpdf
         sudo ln -s $(which true) /usr/local/bin/checkbashisms
       shell: bash
+
+    - name: "Band aid: install rmarkdown from GitHub in older R"
+      run: |
+        try(
+          if (getRversion() < "3.5" && packageVersion("rmarkdown") < "2.13") {
+            pak::pak("rstudio/rmarkdown")
+          }
+        )
+      shell: Rscript {0}