e2e: Fix site editor test (#88571)

After probably WordPress/gutenberg#59317 made the editor iframe be
considered a button labeled "Edit", a test started failing complaining
about there being two such buttons:

```
    locator.click: Error: strict mode violation: locator('body.block-editor-page').getByRole('button', { name: 'Edit', exact: true }) resolved to 2 elements:
        1) <button type="button" aria-label="Edit" class="component…>…</button> aka getByLabel('Navigation').getByLabel('Edit')
        2) <iframe tabindex="0" role="button" aria-label="Edit" nam…></iframe> aka locator('iframe[name="editor-canvas"]')
```

This fixes it by narrowing the search to the "Navigation" section of the
page.

packages/calypso-e2e/src/lib/components/full-side-editor-nav-sidebar-component.ts

@@ -91,7 +91,10 @@ export class FullSiteEditorNavSidebarComponent {
 	 */
 	async clickNavButtonByExactText( text: string ): Promise< void > {
 		const editorParent = await this.editor.parent();
-		await editorParent.getByRole( 'button', { name: text, exact: true } ).click();
+		await editorParent
+			.getByLabel( 'Navigation' )
+			.getByRole( 'button', { name: text, exact: true } )
+			.click();
 	}
 
 	/**