Command substitution for current working directory

I am currently using command substitution in my `launch.json` file as follows
```
{
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"version": "0.2.0",
"configurations": [
{
"name": "test_template",
"type": "gdb",
"request": "launch",
"target": "${command:cmake.launchTargetPath}",
"arguments": "",
"stopAtEntry": false,
"cwd":"${command:cmake.launchTargetDirectory}",
}
]
}
```

The issue I have is that, in my cmake file, I am setting the working directory for my tests as `${CMAKE_CURRENT_SOURCE_DIR}`.
Now in this case `cmake.launchTargetDirectory` doesn't really help me as it points to the path in the build directory and not in the source tree, resulting in my test failing as it relies on relative paths to find input data. Is there an easy way to access `CMAKE_CURRENT_SOURCE_DIR` in the command substitution path ?

Start with the launch.json configuration and the cmake.launchTargetDirectory command substitution described in the issue; trace how command substitutions are resolved and how CMake exposes CMAKE_CURRENT_SOURCE_DIR. Done means a launch configuration can resolve the source-tree working directory for tests that use relative input paths, with coverage for the new substitution behavior.

Written by the indexing model from the issue text.