Fix venv-creation in Makefile

Makefile

@@ -15,15 +15,15 @@ test: $(DEV)
 	$(PYTHON) -m pytest --color=yes tests
 mypy: $(DEV)
 	$(PYTHON) -m mypy --strict --show-error-context --pretty src tests
-$(DEV): env
+$(DEV): $(VENV)
 	$(PYTHON) -m pip install -e .[dev]
 
 .PHONY: build
 build: dist/
 dist/: $(BUILD) src/ README.* LICENSE
 	rm -rf dist
 	$(PYTHON) -m build
-$(BUILD): env
+$(BUILD): $(VENV)
 	$(PYTHON) -m pip install build
 
 .PHONY: clean