postgresql: refactor to pass jitSupport/llvm via scope instead of passthru

This makes it less error-prone to use the llvm package in extensions, because
it will always match the package used by the postgresql derivation itself.

Previously, you could've accidentally used llvm instead of postgresql.llvm
with a different result.

nixos/modules/services/databases/postgresql.nix

@@ -14,7 +14,7 @@ let
       #     package = pkgs.postgresql_<major>;
       #   };
       # works.
-      base = if cfg.enableJIT && !cfg.package.jitSupport then cfg.package.withJIT else cfg.package;
+      base = if cfg.enableJIT then cfg.package.withJIT else cfg.package;
     in
     if cfg.extraPlugins == []
       then base