Make the review-shell purer

The purpose of this `env` is to add the binaries to the `PATH`.

The benefit of making it purer is that unwrapped programs missing
dependencies may fail.

As a added benefit this dramatically reduces the collision warnings from `buildEnv`

`warning: collision between '/nix/store/82icynaidvzyic2g349cqi4p9a5d4aa2-vimplugin-ai.vim-2023-10-03/doc/tags' and '/nix/store/qc4bdwi6zhk4r1vzbcxrnwrrjnm33a6g-vimplugin-CheckAttach-2019-05-08/doc/tags'`

nixpkgs_review/nix.py

@@ -312,6 +312,7 @@ def write_shell_expression(
   env = buildEnv {
     name = "env";
     inherit paths;
+    pathsToLink = [ "/bin" ];
     ignoreCollisions = true;
   };
 in (import ./nixpkgs { }).mkShell {