R/generate-shell: Use R from current tree, fix missing wget dep.

Without this change, the 'R' version used might be the wrong
one which changes the package lists used.

Fixes #19530.

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

@@ -1,10 +1,11 @@
-with import <nixpkgs> {};
+with import ../../.. {};
 
 stdenv.mkDerivation {
   name = "generate-r-packages-shell";
 
   buildCommand = "exit 1";
 
+  buildInputs = [ wget ];
   nativeBuildInputs = [ 
     (rWrapper.override {
       packages = with rPackages; [