MAINT: Verify fork logic for 3.12

[HaoZeke]

Essentially from the flaky CI in #1445 reproduced here:

test/test_gh_pages.py: 64 warnings
test/test_publish.py: 144 warnings
test/test_run.py: 20 warnings
test/test_util.py: 8 warnings
  /opt/hostedtoolcache/Python/3.12.8/x64/lib/python3.12/multiprocessing/popen_fork.py:66: DeprecationWarning: This process (pid=6217) is multi-threaded, use of fork() may lead to deadlocks in the child.
    self.pid = os.fork()

[HaoZeke]

FWIW this is likely the cause of:

=================================== FAILURES ===================================
_______________________________ test_run_publish _______________________________
capfd = <_pytest.capture.CaptureFixture object at 0x7f389aa4e810>
basic_conf = ('/tmp/pytest-of-runner/pytest-0/test_run_publish0', '/home/runner/work/asv/asv/test', <asv.config.Config object at 0x7f389aa4fb60>, '/tmp/pytest-of-runner/pytest-0/test_run_publish0/asv-machine.json')
    @pytest.mark.skipif(
        HAS_PYPY or (not HAS_VIRTUALENV) or WIN,
        reason="Flaky on pypy and windows, needs virtualenv",
    )
    def test_run_publish(capfd, basic_conf):
        tmpdir, local, conf, machine_file = basic_conf
        tmpdir = util.long_path(tmpdir)
    
        conf.matrix = {
            "req": dict(conf.matrix),
            "env": {"SOME_TEST_VAR": ["1"]},
        }
    
        # Tests a typical complete run/publish workflow
>       ret = tools.run_asv_with_conf(conf, 'run', f"{util.git_default_branch()}", '--steps=2',
                                      '--quick', '--show-stderr', '--profile',
                                      '-a', 'warmup_time=0',
                                      '--durations=5',
                                      _machine_file=machine_file)
/home/runner/work/asv/asv/test/test_workflow.py:89: 
_ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ 
/home/runner/work/asv/asv/test/tools.py:175: in run_asv_with_conf
    return cls.run_from_conf_args(conf, args, **kwargs)
/home/runner/work/asv/asv/asv/commands/run.py:166: in run_from_conf_args
    return cls.run(
/home/runner/work/asv/asv/asv/commands/run.py:278: in run
    Setup.perform_setup(environments, parallel=parallel)
/home/runner/work/asv/asv/asv/commands/setup.py:82: in perform_setup
    list(map(_create, environments))
/home/runner/work/asv/asv/asv/commands/setup.py:14: in _create
    env.create()
/home/runner/work/asv/asv/asv/environment.py:754: in create
    self._setup()
_ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ 
self = <asv.plugins.virtualenv.Virtualenv object at 0x7f389aa9b980>
    def _setup(self):
        """
        Setup the environment on disk using virtualenv.
        Then, all of the requirements are installed into
        it using `pip install`.
        """
        env = dict(os.environ)
        env.update(self.build_env_vars)
    
        log.info(f"Creating virtualenv for {self.name}")
>       util.check_call([
            sys.executable,
            "-mvirtualenv",
            "--wheel=bundle",
            "--setuptools=bundle",
            "-p",
            self._executable,
            self._path], env=env)
E       asv.util.ProcessError: Command '/opt/hostedtoolcache/Python/3.12.8/x64/bin/python -mvirtualenv --wheel=bundle --setuptools=bundle -p /opt/hostedtoolcache/Python/3.12.8/x64/bin/python3.12 /tmp/pytest-of-runner/pytest-0/test_run_publish0/env/7719a3cfc6d3dbe18f7664590eb9b261' returned non-zero exit status 1
/home/runner/work/asv/asv/asv/plugins/virtualenv.py:133: ProcessError
----------------------------- Captured stdout call -----------------------------
[100.00%] ··· No `environment_type` specified in asv.conf.json. This will be required in the future.
[100.00%] ··· Creating environments
[100.00%] ···· Error running /opt/hostedtoolcache/Python/3.12.8/x64/bin/python -mvirtualenv --wheel=bundle --setuptools=bundle -p /opt/hostedtoolcache/Python/3.12.8/x64/bin/python3.12 /tmp/pytest-of-runner/pytest-0/test_run_publish0/env/7719a3cfc6d3dbe18f7664590eb9b261 (exit status 1)
               STDOUT -------->
               
               STDERR -------->
               /opt/hostedtoolcache/Python/3.12.8/x64/bin/python: No module named virtualenv
[100.00%] ···· Failure creating environment for virtualenv-py3.12-asv_dummy_test_package_1-asv_dummy_test_package_20.3.8-SOME_TEST_VAR1