Remove assumption that MSBuild controls AssemblyLoadContext.Default

[rainersigwald]

MSBuild has several explicit references to AssemblyLoadContext.Default. That prevents an application from using the MSBuild API in a disposable (or otherwise managed) context.

msbuild/src/Shared/TaskEngineAssemblyResolver.cs

Line 126 in 414393f

return AssemblyLoadContext.Default.LoadFromAssemblyPath(_taskAssemblyFile);

msbuild/src/Shared/MSBuildLoadContext.cs

Lines 82 to 85 in 414393f

	if (FileSystems.Default.FileExists(assemblyNameInExecutableDirectory))
	{
	return AssemblyLoadContext.Default.LoadFromAssemblyPath(assemblyNameInExecutableDirectory);
	}

This one is potentially OK since it's only to support an escape-hatch feature, but we can also consider removing it:

msbuild/src/Shared/CoreCLRAssemblyLoader.cs

Lines 65 to 83 in 414393f

	private Assembly LoadUsingLegacyDefaultContext(string fullPath)
	{
	lock (_guard)
	{
	if (!_resolvingHandlerHookedUp)
	{
	AssemblyLoadContext.Default.Resolving += TryResolveAssembly;
	_resolvingHandlerHookedUp = true;
	}
	Assembly assembly;
	if (_pathsToAssemblies.TryGetValue(fullPath, out assembly))
	{
	return assembly;
	}
	return LoadAndCache(AssemblyLoadContext.Default, fullPath);
	}
	}