builder: Use venv for butterfly validation

builder/test/tests.sh

@@ -27,7 +27,11 @@ tcpdump --version
 monkey --version
 pigpiod -v
 i2cdetect -V
+# butterfly uses its own venv, so we honor that
+. /root/butterfly_env/bin/activate
 butterfly -h
+# deactivate venv as needed
+deactivate
 espeak --version
 mjpg_streamer --version