Fix the environment setup to correctly reuse the installed environment.

Before this PR, if the virtual environment already existed, an attempt would be made to reactivate it instead of to create it; but an incorrect path meant this would fail

Makefile

@@ -20,7 +20,7 @@ help: ## me
 VENV = .venv
 .PHONY: virtualenv
 virtualenv: ## create an activate a virtual env
-	test -f venv/bin/activate || $(PYTHON) -m venv $(VENV)
+	test -f $(VENV)/bin/activate || $(PYTHON) -m venv $(VENV)
 	echo "Run 'source $(VENV)/bin/activate' to activate the virtual environment"