[ci] Fail CI if missing override instead of just warning

[ferdymercury]

Currently, many override warnings remain unnoticed since the gcc-problem-matcher of the GitHub CI does not work correctly, and it's also easy to forget those annotations if you do not visit the "Files changed" tab.

If some devs later use dev=ON and testing=ON, this leads to failures for compilers that auto-enable -Winconsistent-missing-override.

To prevent this, we enable the suggest-override warning in the build of debian with dev=ON instead of in the one from alma, this way we are sure of always failing if some does a PR that forgets an override, forcing pull-requesters to fix their issues before merging.

Within this PR: also fix missing overrides uncovered by turning ON this flag in the dev=ON build.

[ferdymercury]

@pcanal I see a fatal error: module file '/github/home/ROOT-CI/build/lib/MultiProc.pcm' is out of date and needs to be rebuilt: module file out of date in https://github.com/root-project/root/actions/runs/15753057412/job/44402174612?pr=19092

[github-actions]

Test Results

    18 files      18 suites   3d 5h 28m 55s ⏱️
 3 007 tests  3 004 ✅ 0 💤  3 ❌
52 855 runs  52 832 ✅ 0 💤 23 ❌

For more details on these failures, see this check.

Results for commit 835021f.

♻️ This comment has been updated with latest results.