MSBuild fails because `VCTargetsPath` is not set

I cannot use Disasmo in my solution, because of build failures in a referenced VC++ project:
```
MSBuild version 17.9.8+b34f75857 for .NET
Determining projects to restore...
Restored C:\Users\de\Source\my-awesome-project\CTP1bLibrary\CTP1bLibrary.csproj (in 373 ms).
Restored C:\Users\de\Source\my-awesome-project\Scanner2Mat\Scanner2Mat.csproj (in 373 ms).
3 of 5 projects are up-to-date for restore.
C:\Users\de\Source\my-awesome-project\MatIRILib\MatIRILib.vcxproj(20,3): error MSB4019: The imported project "C:\Microsoft.Cpp.Default.props" was not found. Confirm that the expression in the Import declaration "\Microsoft.Cpp.Default.props" is correct, and that the file exists on disk.
```

Apparently MSBuild is looking for `Microsoft.Cpp.Default.props` in the wrong directory. The import in the project file is
``. It appears that `VCTargetsPath` is not set correctly when Disasmo tries to build my solution.

# Repro
1. Create a C# 8 project
2. Add a reference to a VC++ project.