Workaround failing checks on Debian/Ubuntu (setuptools 60+)

Debian patches setuptools/distutils, but not the distutils copy embedded
in setuptools, so pypa/pip#6264 results in

    AttributeError: install_layout

Force SETUPTOOLS_USE_DISTUTILS=stdlib until (if ever) the issue is resolved.

Makefile

@@ -93,6 +93,11 @@ define VENV_CREATE_SYSTEM_SITE_PACKAGES
 	touch $(VENV_SYSTEM_SITE_PACKAGES)
 endef
 
+# workaround for https://bugs.debian.org/cgi-bin/bugreport.cgi?bug=1003252 and/or https://github.com/pypa/pip/issues/6264
+ifdef VENV_USE_SYSTEM_SITE_PACKAGES
+$(VENV_DONE): export SETUPTOOLS_USE_DISTUTILS := stdlib
+endif
+
 $(VENV_DONE): $(MAKEFILE_LIST) setup.py setup.cfg pyproject.toml
 	$(if $(VENV_USE_SYSTEM_SITE_PACKAGES),$(VENV_CREATE_SYSTEM_SITE_PACKAGES),$(VENV_CREATE))
 	$(VENV_PYTHON) -m pip install -e $(VENV_PIP_INSTALL)