fix(flights): keep ground-snap sampling after the clock steps back (#1399)

Recover aircraft ground-height sampling after backward wall-clock changes while preserving sampling budgets.

Contribution by @quanticsoul4772.

changelog.d/1399-ground-snap-clock-step.md

@@ -0,0 +1,3 @@
+- Aircraft on the ground keep getting ground-height samples after the
+  computer's clock steps back, instead of waiting for it to catch up
+  (quanticsoul4772, #1399).