fix: cells backend URL not properly mapped when multibackend ON - WPB-20737 🍒

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This PR was automatically cherry-picked based on the following PR:

• fix: cells backend URL not properly mapped when multibackend ON - WPB-20737 #3674

Original PR description:

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WPB-20737 [iOS] Wire cells on non production backends is broken

Issue

Context: When multibackend is ON and we switch backend to staging (or any other environment), the cells URL always resolve to prod.

Issue: When multibackend is ON, we still use the legacy backend environment which is set to production so the cells URL is not properly mapped.

Solution: Use the new backend environment if any (when multibackend enabled) otherwise use the legacy one (multibackend disabled)

Testing

switch backend from the developer menu with multibackend enabled, make sure cells URL is correct

Checklist

• Title contains a reference JIRA issue number like [WPB-XXX].
• Description is filled and free of optional paragraphs.
• Adds/updates automated tests.

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UI accessibility checklist

If your PR includes UI changes, please utilize this checklist:

• Make sure you use the API for UI elements that support large fonts.
• All colors are taken from WireDesign.ColorTheme or constructed using WireDesign.BaseColorPalette.
• New UI elements have Accessibility strings for VoiceOver.

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Test Results

1 872 tests   1 845 ✅  2m 8s ⏱️
  299 suites     27 💤
    1 files        0 ❌

Results for commit ff7299e.