install-octopi: Use "virtualenv -p python2" to ensure python2 is selected

Signed-off-by: Kevin O'Connor <kevin@koconnor.net>

scripts/install-ubuntu-18.04.sh

@@ -36,7 +36,7 @@ create_virtualenv()
     report_status "Updating python virtual environment..."
 
     # Create virtualenv if it doesn't already exist
-    [ ! -d ${PYTHONDIR} ] && virtualenv ${PYTHONDIR}
+    [ ! -d ${PYTHONDIR} ] && virtualenv -p python2 ${PYTHONDIR}
 
     # Install/update dependencies
     ${PYTHONDIR}/bin/pip install -r ${SRCDIR}/scripts/klippy-requirements.txt