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/pgvecto-rs.nix

@@ -74,7 +74,7 @@ let
 in
 lib.recurseIntoAttrs (
   lib.concatMapAttrs (n: p: { ${n} = makeTestFor p; }) (
-    lib.filterAttrs (_: p: !p.pkgs.pgvecto-rs.meta.broken) pkgs.postgresqlVersions
+    lib.filterAttrs (_: p: p ? pkgs && !p.pkgs.pgvecto-rs.meta.broken) pkgs.postgresqlVersions
   )
   // {
     passthru.override = p: makeTestFor p;