support using conda

This is desirable because this requires no changes to system python packages, does not depend on a system version of python 3.12 being available, and has 0 interference with projects whose build process depend on python.

BUG: 481880

Test plan:

Don't change cmake options and run

./kde-builder breeze-gtk --no-src --no-include-dependencies --refresh-build --conda-env kde

Since breeze-gtk has python build dependencies, this command working indicates the script works as intended.