R: compile with gfortran8 to match the version of gcc

Fixes a build errors like https://hydra.nixos.org/build/102692469,
where R mixes C++ and Fortran code into one shared library.

pkgs/top-level/all-packages.nix

@@ -14503,6 +14503,7 @@ in
   ### DEVELOPMENT / R MODULES
 
   R = callPackage ../applications/science/math/R {
+    gfortran = gfortran8;
     # TODO: split docs into a separate output
     texLive = texlive.combine {
       inherit (texlive) scheme-small inconsolata helvetic texinfo fancyvrb cm-super;