SCons: Address minor `show_progress` issues

[Repiteo]

• Followup to #98154

Realized a minor regression introduced by the above PR causing node count to not be tracked whatsoever if not displaying a progress bar. Additionally, progress_finish as an SCons action is unnecessary & doesn't catch all cases; it's now handled via atexit like the cache cleaning

[hpvb]

LGTM

[dustdfg]

Is it about a fact that [Initial build] disappeared recently or something else?

[Repiteo]

Is it about a fact that [Initial build] disappeared recently or something else?

That being removed was deliberate, but this PR makes sure that it will only happen on an initial build

[akien-mga]

Needs rebase.

[akien-mga]

Thanks!