[Nix] Fix POETRY_HOME environment variable in shell.nix nix expression

[rszamszur]

• I have searched the issues of this repo and believe that this is not a duplicate.

Describe the bug

Environment variable POETRY_HOME is not the same as overridden pkgs.poetry build. This will cause problems when pkgs.python will be different than the python used by default for pkgs.poetry for specific nixpkgs. Meaning, even though poetry will be installed with correct python version, when using make install target it will use a different abspath to poetry and a different python interpreter. For example with pkgs.python310:

$ poetry env info

Virtualenv
Python:         3.10.4
Implementation: CPython
Path:           NA

System
Platform: linux
OS:       posix
Python:   /nix/store/73jg56dr425ycv9p746arzf13m52q2n3-python3-3.10.4

$ echo $POETRY_HOME
/nix/store/p6n63a3jjcr6kxsyxllh1jqnqk03zbwy-python3.9-poetry-1.1.14

$ readlink -f $(which poetry)
/nix/store/w8nds4z9r8jadj74s5zjp9m28mns8ywv-python3.10-poetry-1.1.14/bin/poetry

$ make install
[install] Begin installing project.
...
...
$ poetry env info

Virtualenv
Python:         3.9.13
Implementation: CPython
Path:           /home/rszamszur/repos/gh/rszamszur/fastapi-mvc/.venv
Valid:          True

System
Platform: linux
OS:       posix
Python:   /nix/store/xpwwghl72bb7f48m51amvqiv1l25pa01-python3-3.9.13

The root cause:
https://github.com/rszamszur/fastapi-mvc/blob/ba351bb8a5e4ee7d8a1188aff301d38e04aeb932/shell.nix#L9-L12

Expected behavior

Environment variable POETRY_HOME should point to overridden pkgs.poetry build.

To Reproduce

Use following nix shell expression:

{ pkgs ? import <nixpkgs> { } }:

pkgs.mkShell {
  buildInputs = [
    pkgs.curl
    pkgs.gnumake
    pkgs.podman
    pkgs.python310
    (pkgs.poetry.override { python = pkgs.python310; })
  ];
  shellHook = ''
    export POETRY_HOME=${pkgs.poetry}
    export POETRY_VIRTUALENVS_IN_PROJECT=true
    unset SOURCE_DATE_EPOCH
  '';
}

Install the project via make install target. And run poetry env info.

Environment

• Python version: 3.10.4
• Operating System and version: Linux hades 5.15.56 #1-NixOS SMP Thu Jul 21 19:24:44 UTC 2022 x86_64 GNU/Linux
• fastapi-mvc version: 0.14.0

Additional context

[rszamszur]

While at it, let's make python configurable via --argstr