system-info: Improve diagnostics when layering scout onto soldier

• system-info: Make STEAM_RUNTIME take precedence over os-release

  If we're running the scout runtime inside a soldier container, then we want to be reporting facts about the scout runtime.

• system-info: Show /overrides even if in scout runtime

  This is helpful if we're stacking the scout runtime onto the soldier runtime: both pinned_libs_* and overrides are relevant there.