Rely on environment variables to find Python

[cgravill]

When CMake is locating Python on Windows https://cmake.org/cmake/help/latest/module/FindPython3.html it defaults to searching the registry first. This is awkward behaviour on e.g. GitHub Actions where it has a later version of Python available but wouldn't be used if calling e.g. python -V.

I think it's closer to expectations to respect the environment variables instead where we want to use a precise version for benchmark comparability. That matches the current documentation which instructs put the binaries on the PATH:

https://github.com/microsoft/ADBench/blob/191c5244e7a815984d302ccbc48372137235a9dd/docs/BuildAndTest.md#prerequisites

[tomjaguarpaw]

Does this mean that it ignores the registry entirely?

[cgravill]

With this proposed new setting yes. It completely ignores the registry.

A more friendly option would be to use the CMake LAST setting which first uses environment variables and would then fall back on the registry if there's nothing on PATH.

[awf]

Yep, great fix. Ouch!