Should remove as many hard-coded paths from generated shell files

[EricCousineau-TRI]

At present, I see things like

for workspace in ~/.cache/bazel/_bazel_{user}/{hash}/external/ros2/ros2-linux; do ...

I don't believe we should be using absolute paths like this. I suggest we use ${BASH_SOURCE} or something like $(readlink -f ${BASH_SOURCE}) instead.