Return null from AssemblyLoadContext on unknown

This allows the runtime to handle attempting to load the file in another ALC,
rather than forcing it to attempt to load it in the Default context.

Author: rainersigwald

src/GitVersionTask.MsBuild/LibGit2Sharp/GitLoaderContext.cs

@@ -26,7 +26,7 @@ protected override Assembly Load(AssemblyName assemblyName)
                 return LoadFromAssemblyPath(path);
             }
 
-            return Default.LoadFromAssemblyName(assemblyName);
+            return null;
         }
 
         protected override IntPtr LoadUnmanagedDll(string unmanagedDllName)