Merge pull request #193669 from teto/add-pandoc-to-rmarkdown

rPackages.rmarkdown: add pandoc as buildInput

pkgs/development/r-modules/default.nix

···
     ncdfFlow = [ pkgs.zlib.dev ];
     proj4 = [ pkgs.proj.dev ];
     rtmpt = [ pkgs.gsl ];
+    rmarkdown = [ pkgs.pandoc ];
     mixcat = [ pkgs.gsl ];
     libstableR = [ pkgs.gsl ];
     landsepi = [ pkgs.gsl ];