fix runtox.sh for GNU implementation of tr

Author: untitaker

scripts/runtox.sh

@@ -23,4 +23,4 @@ elif [ -n "$AZURE_PYTHON_VERSION" ]; then
     fi
 fi
 
-exec $TOXPATH -e $($TOXPATH -l | grep "$searchstring" | tr '\n' ',') -- "${@:2}"
+exec $TOXPATH -e $($TOXPATH -l | grep "$searchstring" | tr $'\n' ',') -- "${@:2}"