Variable substitution stopped working in version 1.7

### Brief Issue Summary

I am having an issue similar to the one described in https://github.com/microsoft/vscode-cmake-tools/issues/1526. I am trying to set the **cmake.sourceDirectory** using the variable **${buildKit}** as follows:

` "cmake.sourceDirectory": "${workspaceFolder}/project_1/${buildKit}",`

I cannot configure the cmake project and I receive the error shown below.

I was able to later solve this by installing the previous version of cmaketools version. 1.6.0, and the substitution worked fine. I believe something broke in the transition to 1.7

### CMake Tools Log

```
[expand] Invalid variable reference ${buildKit} in string: ${workspaceFolder}/project_1/${buildKit}
[rollbar] "sourceDirectory" is not a directory {"sourceDirectory":"/workspaces/copateneida/project_1/${buildKit}"}
```

### Platform and Versions

- **Operating System**: Ubuntu 20.04
- **CMaketools Version**: 1.7.3