[bazel] Should avoid allowing `/usr/local` to (automatically) leak in

[EricCousineau-TRI]

/usr/local, at least on Ubuntu, means someone custom-built something and exposed it at system-level, and it may end up shadowing / conflicting with sibling and downstream packages. https://github.com/RobotLocomotion/drake-ros/blob/f63677984fccd4da4ab6e2950939346f02f60c9f/bazel_ros2_rules/ros2/resources/ros2bzl/scraping/ament_cmake.py#L83-L84

We should avoid it by default, and let users opt-in to the foot-gun if they really want it. (For Mac, I believe that the whole Homebrew system may mean this is less foot-gun-y, so maybe we special case that.)

\cc @sloretz