postgresql: refactor to remove "this" argument

This was proposed by abbradar in #150801, but left out of the follow up PR
#221851 by Ma27 to reduce the size of the diff. Compared to the initial
proposal this includes the callPackage call in the recursion, which avoids
breaking the withJIT/withoutJIT helpers.

In terms of nixpkgs, this is a pure refactor, no derivations change. However,
this makes downstream expressions like the following possible:

  (postgresql.override { jitSupport = true; }).pkgs.postgis

This would have not worked before without passing another "this" argument,
which is error prone as can be seen in this example:

  https://github.com/PostgREST/postgrest/pull/3222/files

1 files changed, 1 insertions, 1 deletions

diff --git a/nixos/modules/services/databases/postgresql.md b/nixos/modules/services/databases/postgresql.md
index 7d141f12b5dea..3ff1f00fa9cfb 100644
--- a/nixos/modules/services/databases/postgresql.md
+++ b/nixos/modules/services/databases/postgresql.md
@@ -277,7 +277,7 @@ self: super: {
 Here's a recipe on how to override a particular plugin through an overlay:
 ```
 self: super: {
-  postgresql_15 = super.postgresql_15.override { this = self.postgresql_15; } // {
+  postgresql_15 = super.postgresql_15// {
     pkgs = super.postgresql_15.pkgs // {
       pg_repack = super.postgresql_15.pkgs.pg_repack.overrideAttrs (_: {
         name = "pg_repack-v20181024";