use flake8 inside tower env

Makefile

@@ -425,6 +425,9 @@ pep8: reports
 	@(set -o pipefail && $@ | tee reports/$@.report)
 
 flake8: reports
+	@if [ "$(VENV_BASE)" ]; then \
+		. $(VENV_BASE)/tower/bin/activate; \
+	fi; \
 	@$@ --output-file=reports/$@.report
 
 pyflakes: reports