Splitting the executable in a separate output avoids distributing all of
the libraries and documentation, which are not needed when just running
PostgREST.
Reduces closure size from 4.3G to 73.9M for the flake exported packages.
Resolves#4149
This also moves the pin for nixpkgs into flake.lock instead of our
custom file. Even for the classic interface via default.nix, the pin
will be loaded from flake.lock, thus everything stays in-sync.
This just exposes the PostgREST package, not more.
The goal is to avoid duplication, so we're re-using only exports from
default.nix.
Resolves#3026
Supersedes #3105
