stdenv/check-meta: getEnv if the attribute is unset (#72376)

There were two issues:

* builtins.getEnv was called deep into the nixpkgs tree making it hard
  to discover. This is solved by moving the call into
  pkgs/top-level/impure.nix
* when the config was explicitly set by the user to false, it would
  still try and load the environment variable. This meant that it was
  not possible to guarantee the same outcome on two different systems.