[godot] The Preview Skin property of SpineSprite may override the skin set by code.

[misaki-eymard]

Description:
If you apply any skin to thePreview Skin property of a SpineSprite node in the Inspector dock, the skin may take precedence even if you apply another skin to that SpineSprite via the code.

Expected behavior:
If a skin is set by code, it always takes precedence over the preview skin.

Steps to reproduce:

1. Open the example project with spine-godot 3.5.3 or 4.2.2.
2. Open the 03-mix-and-match/mix-and-match.tscn scene.
3. Play the scene without any changes to confirm that the skin appears correctly.

4. Set the default skin to the Preview Skin on the Inspector dock.

5. Play the scene again, then you will see the skin is no longer displayed correctly.

The version of runtime in which this issue was found:
spine-godot 3.5.3 and 4.2.2

This issue was reported on the following thread:
https://esotericsoftware.com/forum/d/26208-godot-spine-sprite%E8%8A%82%E7%82%B9%E7%96%91%E9%97%AE