rosnode: allow rosrun & roslaunch to find script

[gavanderhoorn]

As per subject.

See the commit comment for more info.

Inspired by what rosservice does:

https://github.com/ros/ros_comm/blob/dd78ac8af128bb8eb992d6431bb9f994658ea6ab/tools/rosservice/CMakeLists.txt#L8-L9

I'm not aware of any adverse side-effects, but haven't extensively tried to look for any either.

[gavanderhoorn]

Discovered this while updating rosnode kill in launch file on ROS Answers.