Disable SwiftSyntax prebuilts for UI tests

[Kyle-Ye]

Summary

• disable SwiftSyntax package prebuilts for both UI test xcodebuild invocations
• apply the workaround consistently across iOS and macOS UI test jobs
• document that Swift 6.3 fixes the issue so the workaround can be removed after the Xcode 26.6 upgrade

Context

Swift 6.2 can select SwiftSyntax macro prebuilts that cause architecture or duplicate-link failures in UI test builds. Passing -IDEPackageEnablePrebuilts=NO keeps both baseline recording and OpenSwiftUI UI tests on the same temporary workaround.

Validation

• parsed .github/actions/uitests/action.yml as YAML
• ran git diff --check

[augmentcode]

🤖 Augment PR Summary

Summary: Updates the UI test GitHub Action to run xcodebuild test with -IDEPackageEnablePrebuilts=NO for both baseline recording and UI test jobs.
Why: Works around Swift 6.2 selecting problematic SwiftSyntax macro prebuilts (arch/duplicate-link failures) until Swift 6.3 / the Xcode 26.6 upgrade.

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[augmentcode]

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[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 26.35%. Comparing base (4c426a7) to head (b3c6f8b).
⚠️ Report is 1 commits behind head on main.

Additional details and impacted files

@@            Coverage Diff             @@
##             main     #942      +/-   ##
==========================================
- Coverage   26.36%   26.35%   -0.01%     
==========================================
  Files         723      723              
  Lines       51232    51232              
==========================================
- Hits        13507    13502       -5     
- Misses      37725    37730       +5     

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