treewide: simplify pkgs.stdenv.lib -> pkgs.lib

The library does not depend on stdenv, that `stdenv` exposes `lib` is
an artifact of the ancient origins of nixpkgs.

nixos/tests/sssd-ldap.nix

@@ -10,7 +10,7 @@
   in import ./make-test-python.nix {
     name = "sssd-ldap";
 
-    meta = with pkgs.stdenv.lib.maintainers; {
+    meta = with pkgs.lib.maintainers; {
       maintainers = [ bbigras ];
     };