re-register task provider for new target

[elahehrashedi]

bug fix: #1576

using tasks.json:

{
	"type": "cmake",
	"label": "CMake Build test",
	"command": "build",
	"problemMatcher": [],
	"group": {
		"kind": "build",
		"isDefault": true
	}
}

This code will fix these commands (when "cmake.buildTask": true)

Cmake: build (builds the default task)
Cmake: build target (chooses a task to build)
Tasks: Run Task
Tasks: Run Build Task

When we use Cmake: set build target to change the default build target.
or when we change the build target by right-clicking: