When a PEP 517 project file is present, pip will not install
prerequisites in `site-packages`:
https://pip.pypa.io/en/stable/reference/pip/#pep-517-and-518-support
For the shell hook, this has the consequence that the generated
temporary directory that is added to PYTHONPATH does not contain
`site.py`. As a result, Python does not discover the Python
module. Thus when a user executes nix-shell in a project, they cannot
import the project's Python module.
This change adds the `--no-build-isolation` option to pip when
creating the editable environment, to correctly generate `site.py`,
even when a `pyproject.toml` is present.
e2309df85e