i3lock-fancy: Bugfix, make scrot overwrite file

After commit [1] scrot will no longer overwrite files unless the `-o`
option is given. This commit makes the i3lock-fancy use `scrot -z -o`
instead of just `scrot -o`.

[1]: https://github.com/resurrecting-open-source-projects/scrot/commit/489e3229045962be2f63535ecd70cd15e6e2414d