Switch the openstack-tox-py312* jobs to distro packages

During job development python 3.12 wasn't available and we built it
on demand in debian-bookworm.  Since the release of Ubuntu Noble
python 3.12 is available as the default (packaged) python version.

Let's use that.

Change-Id: Ib77c6b7b872282cfd0a63577c9d738625a0dac6d

zuul.d/jobs.yaml

@@ -364,7 +364,6 @@
     vars:
       tox_envlist: py312
       bindep_profile: test py312
-      python_use_pyenv: True
       python_version: '3.12'
 
 - job: