postgresql: pass through JIT-enabled variant of non-JIT postgres and vice versa

This is useful if your postgresql version is dependant on
`system.stateVersion` and not pinned down manually. Then it's not
necessary to find out which version exactly is in use and define
`package` manually, but just stay with what NixOS provides as default:

    $ nix-instantiate -A postgresql
    /nix/store/82fzmb77mz2b787dgj7mn4a8i4f6l6sn-postgresql-14.7.drv
    $ nix-instantiate -A postgresql_jit
    /nix/store/qsjkb72fcrrfpsszrwbsi9q9wgp39m50-postgresql-14.7.drv
    $ nix-instantiate -A postgresql.withJIT
    /nix/store/qsjkb72fcrrfpsszrwbsi9q9wgp39m50-postgresql-14.7.drv
    $ nix-instantiate -A postgresql.withJIT.withoutJIT
    /nix/store/82fzmb77mz2b787dgj7mn4a8i4f6l6sn-postgresql-14.7.drv

I.e. you can use postgresql with JIT (for complex queries only[1]) like
this:

    services.postgresql = {
      enable = true;
      enableJIT = true;
    };

Performing a new override instead of re-using the `_jit`-variants for
that has the nice property that overlays for the original package apply
to the JIT-enabled variant, i.e.

    with import ./. {
      overlays = [
        (self: super: {
          postgresql = super.postgresql.overrideAttrs (_: { fnord = "snens"; });
        })
      ];
    };
    postgresql.withJIT.fnord

still gives the string `snens` whereas `postgresql_jit` doesn't have the
attribute `fnord` in its derivation.

[1] https://www.postgresql.org/docs/current/runtime-config-query.html#GUC-JIT-ABOVE-COST

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