GHA: Does the new xfun 0.37 require R (>= 3.5.0)

Author: HenrikBengtsson

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

@@ -65,6 +65,7 @@ jobs:
       - name: Install package itself (special case)
         run: |
           install.packages(".", repos = NULL, type = "source")  ## self vignette engine
+	  if (getRversion() < "3.5.0") install.packages("https://cran.r-project.org/src/contrib/Archive/xfun/xfun_0.36.tar.gz")
         shell: Rscript {0}
 
       - name: Session info