CMake build from task not working after Open Folder

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### Brief Issue Summary

As of fixing bug #1234, running CMake build from task is basically working, but not right after Open Folder command.
1. Have a project with CMakeLists.txt in it's root directory.
2. Have this in tasks.json:

```
{
"version": "2.0.0",
"tasks": [
{
"label": "CMake Build",
"type": "cmake",
"command": "build",
"group": "build"
}
]
}
```
3. In launch.json:
```
{
"version": "0.2.0",
"configurations": [
{
...
"preLaunchTask": "CMake Build"
...
}
]
}
```
4. After opening the project using Open Folder command(also, in case it opens automatically at VSCode start), I press F5 key to build project and run it.

### Expected:

Project to be built and run.

### Apparent Behavior:

Got an error in output tab:

```
Error: The cmake task detection didn't contribute a task for the following configuration:
{
"label": "CMake Build",
"type": "cmake",
"command": "build",
"group": "build"
}
The task will be ignored.
```
But after I modify somehow tasks.json(e.g. enter space, delete it and save file), launch works as expected, task found successfully.
If then I open some other folder, and then reopen the project folder again, CMake Build task recognition fail again.

### Platform and Versions

- **Operating System**: Linux Mint 19
- **CMake Version**: 3.5.1
- **VSCode Version**: 1.51.1
- **CMake Tools Extension Version**: v1.5.3
- **Compiler/Toolchain**: arm-none-eabi-gcc 4.8

### Other Notes/Information

Reproduce the failure using the listed tasks.json and launch.json after Open Folder, then compare it with behavior after saving tasks.json. Start by tracing CMake task detection during folder opening; done means the CMake Build task is recognized immediately and F5 builds and runs the project without editing tasks.json.

Written by the indexing model from the issue text.