Fix plugins to make single file assemblies possible

[gsmachado]

I just changed Assembly.GetEntryAssembly().Location to System.AppContext.BaseDirectory (read more here). This will make possible single file assemblies for Neo Nodes.

Actually, the problem with Assembly.GetEntryAssembly().Location is that it doesn't support single file assemblies introduced in .net 6.

Let me know if you have a better way to do it, please. IMHO, this is already the best way (it was even recommended in this issue).

[cschuchardt88]

Also

The GetEntryAssembly method can return null when a managed assembly has been loaded from an unmanaged application. For example, if an unmanaged application creates an instance of a COM component written in C#, a call to the GetEntryAssembly method from the C# component returns null, because the entry point for the process was unmanaged code rather than a managed assembly.