Poor ccache hit rate on debug build with coverage set using colcon

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For the navigation2 package on ros2, I think we're encountering some non-determinism that might be causing the miserable hit rates for our debug build, as opposed to the 100% rate we have for release builds. I'm guessing it might have to do with enabling code coverage, but not sure how. Here are some logs for comparison, with ccache logs in the artifacts tab and the statistics are shown in the CCache Stats step that comes after Build Workspace | /opt/overlay_ws:

• release (100% hit rate):
  • https://circleci.com/gh/ros-planning/...
  • https://2260-135363400-gh.circle-arti...
• debug (14% hit rate):
  • https://circleci.com/gh/ros-planning/...
  • https://2261-135363400-gh.circle-arti...