nixosTests.postgresql.*: fix eval

The `libpq` attribute from `postgresqlVersions` doesn't have a `pkgs`
attribute with extensions in it. Filtering for postgresql server
packages only fixes the problem.

Reported in https://github.com/NixOS/nixpkgs/pull/359659#issuecomment-2601045449

nixos/tests/postgresql/postgresql-tls-client-cert.nix

@@ -129,7 +129,9 @@ let
     };
 in
 lib.recurseIntoAttrs (
-  lib.concatMapAttrs (n: p: { ${n} = makeTestFor p; }) pkgs.postgresqlVersions
+  lib.concatMapAttrs (n: p: { ${n} = makeTestFor p; }) (
+    lib.filterAttrs (_: p: p ? pkgs) pkgs.postgresqlVersions
+  )
   // {
     passthru.override = p: makeTestFor p;
   }