r-modules: always set meta.platforms when meta.hydraPlatforms is set, too

By default, we take both values from the R derivation.

pkgs/development/r-modules/default.nix

@@ -31,6 +31,7 @@ let
     propagatedBuildInputs = depends;
     nativeBuildInputs = depends;
     meta.homepage = "http://cran.r-project.org/web/packages/${name}/";
+    meta.platforms = R.meta.platforms;
     meta.hydraPlatforms = hydraPlatforms;
     meta.broken = broken;
   });