Navigator: fix isInitial logic

[ciampo]

What?

Discovered while working on #65523

Set the isInitial flag to false in the internal Navigator reducer only when effectively navigating to a new screen.

Why?

Previously, the isInitial flag was set to false on every navigation, even the ones that were not leading to effectively navigating to a new screen. This was causing unwanted side effects, like the one flagged in #65523

How?

The isInitial flag is set to false only when navigating to a new location.

Testing Instructions

In Storybook:

• Apply the following diff

diff --git a/packages/components/src/navigator/stories/index.story.tsx b/packages/components/src/navigator/stories/index.story.tsx
index 30b9c71a36..8d4881a563 100644
--- a/packages/components/src/navigator/stories/index.story.tsx
+++ b/packages/components/src/navigator/stories/index.story.tsx
@@ -64,6 +64,10 @@ export const Default: StoryObj< typeof NavigatorProvider > = {
 					<h2>This is the home screen.</h2>
 
 					<VStack alignment="left">
+						<NavigatorButton variant="primary" path="/">
+							Go to home screen.
+						</NavigatorButton>
+
 						<NavigatorButton variant="primary" path="/child">
 							Go to child screen.
 						</NavigatorButton>

• Load the default Storybook example for Navigator
• Click on the "Go to home screen" button
• Make sure the screen doesn't animate

In the editor:

• Open site editor
• Open the global styles sidebar
• Make sure the initial screen doesn't animate

Screenshots or screencast

Before (trunk)	After (this PR)
Kapture.2024-09-20.at.14.24.56.mp4	Kapture.2024-09-20.at.14.26.11.mp4

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Co-authored-by: ciampo <mciampini@git.wordpress.org>

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