Page Animations not inverted when navigating back

**Describe the bug**
When using the provided example to implement custom page transitions: https://github.com/felangel/flow_builder/issues/29 the animation for "forward" and "backward" navigation remains the same.
This only happens when handling the back navigation with the FlowBuilder state (context.flow().update...), but the animation behaves correctly if I use `Navigator.pop(context);`

This is a snipped of my `onGeneratePages`, where I tested the back navigation from `OnboardingFlowPage.commuteDays` to `OnboardingFlowPage.enterAddress` by calling `context.flow.update((state) => OnboardingInitial(page: OnboardingFlowPage.enterAddress)`.

```
case OnboardingFlowPage.enterAddress:
return [
CustomPage.createWithoutAnalytics(
EnterAddressFlow(isOnboarding: isOnboarding))
];
case OnboardingFlowPage.commuteDays:
return [
CustomPage.createWithoutAnalytics(
EnterAddressFlow(isOnboarding: isOnboarding)),
CustomPage.createWithAnalytics(
CommuteDaysPage(isOnboarding: isOnboarding)),
];
```

**To Reproduce**
Steps to reproduce the behavior:
Implement the given example with a custom page transition and do a back navigation by calling `context.flow().update()`.

**Expected behavior**
A clear and concise description of what you expected to happen.
Inverted animation plays

**Screenshots**
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