Fixed nuget packages get lost with Unity 6 (#743)

In Unity 6 it can happen that multiple asset import worker processes get started at the same time then this will lead to undefined behavior because session data is new for each worker process, so it will execute the code in parallel multiple times.

Author: tgoessler

src/NuGetForUnity/Editor/OnLoadNugetPackageRestorer.cs

@@ -15,6 +15,13 @@ public static class OnLoadNugetPackageRestorer
         /// </summary>
         static OnLoadNugetPackageRestorer()
         {
+    #if UNITY_6000_0_OR_NEWER
+            if (AssetDatabase.IsAssetImportWorkerProcess())
+            {
+                return;
+            }
+    #endif
+            
             if (SessionState.GetBool("NugetForUnity.FirstProjectOpen", false))
             {
                 return;