Resolve system variables in the defaultInterpreterPath entry

[kimadeline]

Issue from #18207, fixed by #18215

[kimadeline]

Repro steps:

• Have a workspace with Python files and a .code-workspace file
• Create a subfolder in the workspace, and then create a virtual environment in that subfolder (any flavor of Python 3 works): cd subfolder; python3 -m venv .venv
• Add the following to the .code-workspace file:

{
    "settings": {
        "python.defaultInterpreterPath": "${workspaceFolder}/path/to/python.exe",
    }
}

On windows the path to the python executable will be something like ${workspaceFolder}/<folder>/.venv/Scripts/python.exe, while on Unix it will be ${workspaceFolder}/<folder>/.venv/bin/python.exe

• Open the workspace and activate the extension
• Run the Select Interpreter command, and verify that the path in the Use Python from 'python.defaultInterpreterPath' setting entry is a valid path that contains no system variables
• Select that entry, and verify that it is a usable Python interpreter (the extension works, and operations like linting or formatting work)

[rchiodo]

/verified