[CI] fix macOS task

.cirrus.yml

@@ -32,15 +32,16 @@ macOS_task:
     image: ghcr.io/cirruslabs/macos-runner:sonoma
   env:
     PATH: ${HOME}/.pyenv/shims:${PATH}
+    PYTHON: 3:latest
   brew_update_script:
     - brew update
   brew_install_script:
     # Per the pyenv homebrew recommendations.
     #  https://github.com/pyenv/pyenv/wiki#suggested-build-environment
     - brew install openssl readline sqlite3 xz zlib pyenv git
   pyenv_install_script:
-    - pyenv install 3:latest
-    - pyenv global 3:latest
+    - pyenv install ${PYTHON}
+    - pyenv global 3
     - pyenv rehash
   pip_install_script:
     - pip install -U --upgrade-strategy eager pip 'setuptools>61'