R: don't recurse into the rPackages set to conserve memory and CPU time

The addition of CRAN has made "nix-env -qa \*" run ~30% longer than before [1].

[1] https://github.com/NixOS/nixpkgs/commit/0d264c17616646a7d68e0a465fdc0f335e0d2fb3

pkgs/top-level/all-packages.nix

@@ -6437,10 +6437,10 @@ let
     packages = [];
   };
 
-  rPackages = recurseIntoAttrs (import ../development/r-modules/cran-packages.nix {
+  rPackages = import ../development/r-modules/cran-packages.nix {
     inherit pkgs;
     overrides = (config.rPackageOverrides or (p: {})) pkgs;
-  });
+  };
 
   ### SERVERS