Fix build-test target

Author: cderici

Makefile

@@ -41,13 +41,13 @@ docs:
 	tox -e docs
 
 .PHONY: build-test
-build-test:
+build-test: .tox
 	rm -rf venv
-	python -m venv venv
+	$(PY) -m venv venv
 	. venv/bin/activate
 	$(PY) setup.py sdist
 	pip install dist/juju-${VERSION}.tar.gz
-	python3 -c "from juju.controller import Controller"
+	$(PY) -c "from juju.controller import Controller"
 	rm dist/juju-${VERSION}.tar.gz
 
 .PHONY: release