pythonPackages.pipBuildHook: do not build in an isolated environment

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.

pkgs/development/interpreters/python/hooks/pip-build-hook.sh

@@ -24,7 +24,8 @@ pipShellHook() {
       export PATH="$tmp_path/bin:$PATH"
       export PYTHONPATH="$tmp_path/@pythonSitePackages@:$PYTHONPATH"
       mkdir -p "$tmp_path/@pythonSitePackages@"
-      @pythonInterpreter@ -m pip install -e . --prefix "$tmp_path" >&2
+      @pythonInterpreter@ -m pip install -e . --prefix "$tmp_path" \
+         --no-build-isolation >&2
     fi
 
     runHook postShellHook