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/postgresql.nix

@@ -23,7 +23,7 @@ let
   '';
   make-postgresql-test = postgresql-name: postgresql-package: backup-all: makeTest {
     name = postgresql-name;
-    meta = with pkgs.stdenv.lib.maintainers; {
+    meta = with pkgs.lib.maintainers; {
       maintainers = [ zagy ];
     };