Isolate Godot NuGet restore from Unity package cache

[joshua-spacetime]

Description of Changes

A follow-on to #5133 that fixes another Godot flake that was observed after #5133 merged. Specifically a C# SDK regen flake where the Godot package restore path reused sdks/csharp/packages, which is also the Unity-visible package cache.

During cargo regen csharp dlls, the Godot flow previously did this:

• cleared sdks/csharp/packages/godotsharp
• cleared sdks/csharp/obj~/godot
• restored the Godot project back into sdks/csharp/packages
• packed the Godot project using that same Unity-visible package area

That meant a Godot-only cleanup could delete package content under the Unity SDK package cache. If a previous GodotSharp restore was partial or stale, regen tried to fix it by deleting packages/godotsharp, but that path is visible to Unity and can cause dirty/missing package content during regen.

With this change, Godot restore state is now isolated under sdks/csharp/obj~/godot/packages, and the regen flow becomes:

• clear only sdks/csharp/obj~/godot
• restore the Godot project into sdks/csharp/obj~/godot/packages
• pack the Godot project under the same path

Clearing obj~/godot now removes both Godot intermediates and Godot-only NuGet packages, while leaving the normal Unity SDK package cache under sdks/csharp/packages untouched.

API and ABI breaking changes

N/A

Expected complexity level and risk

1

Testing

• unity-testsuite passes

[bfops]

SGTM thank you!