Cherry-pick "Add CheckerGccPlugins to AthSimulationExternals, main branch (2024.05.08.)" into 2.1.16-patches (2024.06.19.)
One more thing for ATLASSIM-6058, which I forgot about earlier.
See merge request !1126 (merged)
(cherry picked from commit 14af9994)
- 829e7b2f Add CheckerGccPlugins to AthSimulationExternals.