This way we benefit from NixOS' binary cache to deliver GHC for us and
don't need to cache it ourselves.
This will become relevant once we do that for more platforms.
We have fixed the static build of PostgreSQL upstream, so we don't need
our separate overlay anymore. One more step towards building a static
executable on other platforms.
Instead of rolling our own, we can use some tooling from nix / nixpkgs.
We drop the "statically linked" check, because our goal is to compile
mostly-static executables to darwin, too. However, those will never be
fully static, because they always link to the platform's libc.
The enable-executable-static flag is set by default, so doesn't make a difference.
The pkg-config improvement for libpq was merged upstream, so we can use the same
here already.
This makes the static build fail in case any references to the nix store
are left over. Those will increase the closure size of the nix derivation
massively and lead to a huge docker image.
At the same time, those references will not be functional on non-nix systems,
to which the static executable is distributed, anyway.
