use mkPoetryEnv from out-of-tree poetry2nix

[aycanirican]

Fixes nix develop which was throwing an error since we were still using mkPoetryEnv from nixpkgs:

error:
       … while calling the 'derivationStrict' builtin

         at //builtin/derivation.nix:9:12: (source not available)

       … while evaluating derivation 'nix-shell'
         whose name attribute is located at /nix/store/4fgs7yzsy2dqnjw8j42qlp9i1vgarzy0-source/pkgs/stdenv/generic/make-derivation.nix:348:7

       … while evaluating attribute 'buildInputs' of derivation 'nix-shell'

         at /nix/store/4fgs7yzsy2dqnjw8j42qlp9i1vgarzy0-source/pkgs/stdenv/generic/make-derivation.nix:395:7:

          394|       depsHostHost                = elemAt (elemAt dependencies 1) 0;
          395|       buildInputs                 = elemAt (elemAt dependencies 1) 1;
             |       ^
          396|       depsTargetTarget            = elemAt (elemAt dependencies 2) 0;

       error: poetry2nix is now maintained out-of-tree. Please use https://github.com/nix-community/poetry2nix/

[farcaller]

Thanks for a fix!