C#: Load assemblies as collectible only in the Godot editor

We use collectible AssemblyLoadContexts as that's the only way to allow
reloading assemblies after building. However, collectible assemblies
have some restrictions:

- https://learn.microsoft.com/en-us/dotnet/framework/reflection-and-codedom/collectible-assemblies#restrictions-on-collectible-assemblies

Those restrictions can cause issues with third-party code, such as some
mocking libraries.

In order to work around this problem, we're going to load assemblies
as collectible only in Godot editor, and not when running games.
These issues will still exist in the editor, but this will be enough
for some users.

modules/mono/glue/GodotSharp/GodotPlugins/PluginLoadContext.cs

@@ -15,8 +15,8 @@ namespace GodotPlugins
         public string? AssemblyLoadedPath { get; private set; }
 
         public PluginLoadContext(string pluginPath, ICollection<string> sharedAssemblies,
-            AssemblyLoadContext mainLoadContext)
-            : base(isCollectible: true)
+            AssemblyLoadContext mainLoadContext, bool isCollectible)
+            : base(isCollectible)
         {
             _resolver = new AssemblyDependencyResolver(pluginPath);
             _sharedAssemblies = sharedAssemblies;