Fix for system with proxy setting

in original code: stack.sh:~170
 170     if [[ "$SHELL_AFTER_RUN" != "no" ]]; then
 171         exec sudo -u $STACK_USER  bash -l -c "set -e; bash stack.sh; bash"
 172     else
 173         exec sudo -u $STACK_USER bash -l -c "set -e; source stack.sh"
 174     fi

It is unlikely that any env variable is passed to next step.
So I add a few piece of code and it works for me.