postgresql: return wrapper from withoutJIT passthru

Before this change, we would return:
- The postgresql derivation itself from `postgresql.withoutJIT`,
- A wrapper without any extensions at all, from
`postgresql_jit.withoutJIT`.

This changes makes it that every call to `.withJIT` and `.withoutJIT`
returns a wrapper and never the base derivation. More consistent and
easier to reason about.

(cherry picked from commit 5cf857ed82)

pkgs/servers/sql/postgresql/generic.nix

@@ -561,7 +561,7 @@ let
           psqlSchema = lib.versions.major version;
 
           withJIT = if jitSupport then this.withPackages (_: [ this.jit ]) else null;
-          withoutJIT = this;
+          withoutJIT = this.withPackages (_: [ ]);
 
           pkgs =
             let