Use correct build-backend for poetry-core

[SuperSandro2000]

Description

This is necessary to fix the build for NixOS. NixOS/nixpkgs@ce4b26f

Related issues

Type of change

• Bug fix (non-breaking change which fixes an issue)
• New feature (non-breaking change which adds functionality)
• Breaking change (fix or feature that would cause existing functionality to not work as expected)
• CI / Tests update
• Documentation update

How has this been tested

• I have added new tests where required
• I have run nox -s tests locally and passed
• I have tested this feature in a python script

[mweinelt]

This is necessary to fix the build for NixOS.

This is necessary to fix the build. Because I believe pulling in poetry-core and accessing poetry.masonry is an impurity, that can only work when poetry itself is in the environment.