rPackages: generate-shell.nix works with --pure

generate-R-packages.R fails without these changes.

Without `cacert`, all wget/curl calls will fail
with an error about no valid certificates.

Without `nix`, calling `nix-hash` fails.

An impure nix-shell can get away with not adding
these if the system NIXPKGS version is the same
as the version of NIXPKGS used
with `nix-shell generate-shell.nix`

pkgs/development/r-modules/generate-shell.nix

@@ -5,7 +5,11 @@ stdenv.mkDerivation {
 
   buildCommand = "exit 1";
 
-  buildInputs = [ wget ];
+  buildInputs = [
+    wget
+    cacert
+    nix
+  ];
 
   nativeBuildInputs = [
     (rWrapper.override {