VS: CMake does not find portable instance

Repro:

1. Install Visual Studio Build Tools
2. Copy the contents of the install directory (e.g. c:\program files (x86)\Microsoft Visual Studio\2019\BuildTools) to a version-controlled location, e.g. c:\depot\buildtools
3. cmake -DCMAKE_GENERATOR_INSTANCE=c:\depot\buildtools

Expected result: cmake finds the build tools in the specified location

Actual result:

• cmake doesn't even look in the specified location (confirmed with procmon.exe)
• "could not find specified instance of Visual Studio"

One of our engineering pillars is that we can build and run our software having only a version control client installed on the build machine. It seems that cmake expects Visual Studio to be fully installed, and enforces this by querying C:\ProgramData and the registry. These queries should not be necessary when a specific instance location is explicitly provided.