fix(build): respect .python-version when generating pyenv

[iamllama]

ref: https://forums.ankiweb.net/t/anki-build-failed-because-python-3-14-is-installed-on-my-system-in-addition-to-python-3-13/67484?u=llama

Mirrors what we do in the launcher: https://github.com/ankitects/anki/blob/55691fc254a61e1d982873720b20ee6c4b852111/qt/launcher/src/main.rs?plain=1#L336-L346

before:

DEBUG Acquired lock for `/anki`
DEBUG Ignoring Python version file at `.python-version` due to `--no-config`
DEBUG Using Python request `>=3.9` from `requires-python` metadata
DEBUG Checking for Python environment at `out/pyenv`

after:

DEBUG Acquired lock for `/anki`
DEBUG Using Python request `3.13.5` from explicit request
DEBUG Checking for Python environment at `out/pyenv`
DEBUG The project environment's Python version satisfies the request: `Python 3.13.5`