CMake: prefer Python virtual environment if present

[elgiano]

When auto-detecting python for building docs, prefer a version from a virtual environment if present.
I come to this problem when building flucoma-sc. As I don't want to install flucoma-docs dependencies system-wise, I create a virtual environment in my build folder, and pip install just there. With this PR, the build system detects the virtual env automatically.

[tremblap]

I don't know what @weefuzzy think of this, but personally I use virtual environments by declaring:

set(Python_EXECUTABLE "/Users/pa/flucoma-env/bin/python" CACHE PATH "")

when I cmake the sc repo and boom, it works.

[weefuzzy]

I don't see a compelling reason to bake this preference into the cmake code. Things like Python3_FIND_VIRTUALENV are meant as hints passed by users.

To make it less annoying to set every time you could make a CMakeUserPresets.json and declare a preset that sets this variable for you (along with any other repeatedly needed settings). (At some point we're going to move to using presets widely, and quite a lot of cruft can come out of the cmake code (I hope)).