switch to a `nix build ...` statement instead of the `nix-shell`

[silky]

or, alternatively, switch to using the nix-shell to build the haskell project, instead of/in combination with stack.

there is a suggestion that i could use nix build with something like:

$ nix build --impure --print-out-paths --expr 'let pkgs = import <nixpkgs> {};
in pkgs.fetchgit {
            url      = "https://github.com/FixPlanet/org";
            rev      = "4507755ee4ce43e279995b21b384aef338ee81f3";
            sha256   = "sha256-cQzjlrSzu8m8PsbOe/+YVRqc1y0SR4NlfJZ7KKqqyqU=";
            fetchLFS = true;
          }'
/nix/store/8knh445df1pqlfqggqji9z3p4pxdrr4y-org-4507755

as for the stack+nix scheme; see:

• https://www.tweag.io/blog/2022-06-02-haskell-stack-nix-shell/

the changes here should also be carried through to the CI (and the main readme)