Accommodate virtual envs on Windows that use Unix path hierarchy.

[cr1901]

Pull Request Checklist

• A news fragment is added in news/ describing what is new.
• Test cases added for changed code. I'm not sure how to test this? The relevant test is going to always find python under Scripts.

Describe what you have changed in this PR.

If using (a pip install'ed) pdm with a Python from the MSYS2 project, sometimes it is possible for pdm to fail to find the python binary in a freshly-created virtualenv:

William@DESKTOP-3H1DSBV MINGW64 ~/Projects/FPGA/amaranth/template-fpga
$ pdm install -v
STATUS: Resolving packages from lockfile...
The saved Python interpreter doesn't match the project's requirement. Trying to find another one.
python.use_venv is on, creating a virtualenv for this project...
Cleaning existing target directory C:/msys64/home/William/Projects/FPGA/amaranth/template-fpga/.venv
Run command: ['C:/msys64/mingw64/bin/python.exe', '-m', 'virtualenv',
'C:/msys64/home/William/Projects/FPGA/amaranth/template-fpga/.venv', '-p', 'C:/msys64/mingw64/bin/python.EXE',
'--prompt=template-fpga-3.11', '--no-pip', '--no-setuptools', '--no-wheel']
created virtual environment CPython3.11.5.final.0-64 in 196ms
  creator CPython3Windows(dest=C:/msys64/home/William/Projects/FPGA/amaranth/template-fpga/.venv, clear=False, no_vcs_ignore=False, glo

bal=False)
  activators BashActivator,BatchActivator,FishActivator,NushellActivator,PowerShellActivator,PythonActivator
Virtualenv is created successfully at C:/msys64/home/William/Projects/FPGA/amaranth/template-fpga/.venv
Traceback (most recent call last):
  File "<frozen runpy>", line 198, in _run_module_as_main
  File "<frozen runpy>", line 88, in _run_code
  File "C:/msys64/mingw64/bin/pdm.exe/__main__.py", line 7, in <module>
  File "C:/msys64/home/William/Projects/python/pdm/src/pdm/core.py", line 290, in main
    return Core().main(args or sys.argv[1:])
           ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
  File "C:/msys64/home/William/Projects/python/pdm/src/pdm/core.py", line 208, in main
    raise cast(Exception, err).with_traceback(traceback) from None
  File "C:/msys64/home/William/Projects/python/pdm/src/pdm/core.py", line 203, in main
    self.handle(project, options)
  File "C:/msys64/home/William/Projects/python/pdm/src/pdm/core.py", line 157, in handle
    command.handle(project, options)
  File "C:/msys64/home/William/Projects/python/pdm/src/pdm/cli/commands/install.py", line 95, in handle
    actions.do_sync(
  File "C:/msys64/home/William/Projects/python/pdm/src/pdm/cli/actions.py", line 197, in do_sync
    candidates = resolve_candidates_from_lockfile(project, requirements)
                 ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
  File "C:/msys64/home/William/Projects/python/pdm/src/pdm/cli/actions.py", line 138, in resolve_candidates_from_lockfile
    provider = project.get_provider(for_install=True)
               ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
  File "C:/msys64/home/William/Projects/python/pdm/src/pdm/project/core.py", line 429, in get_provider
    repository = self.get_repository(ignore_compatibility=ignore_compatibility)
                 ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
  File "C:/msys64/home/William/Projects/python/pdm/src/pdm/project/core.py", line 397, in get_repository
    return cls(sources, self.environment, ignore_compatibility=ignore_compatibility)
                        ^^^^^^^^^^^^^^^^
  File "C:/msys64/home/William/Projects/python/pdm/src/pdm/project/core.py", line 280, in environment
    self._environment = self.get_environment()
                        ^^^^^^^^^^^^^^^^^^^^^^
  File "C:/msys64/home/William/Projects/python/pdm/src/pdm/project/core.py", line 259, in get_environment
    if self.config["python.use_venv"] and self.python.get_venv() is not None
                                          ^^^^^^^^^^^
  File "C:/msys64/home/William/Projects/python/pdm/src/pdm/project/core.py", line 150, in python
    if self._python.major < 3:
       ^^^^^^^^^^^^^^^^^^
  File "C:/msys64/home/William/Projects/python/pdm/src/pdm/models/python.py", line 57, in major
    return self.version.major
           ^^^^^^^^^^^^
  File "C:/msys64/mingw64/lib/python3.11/functools.py", line 1001, in __get__
    val = self.func(instance)
          ^^^^^^^^^^^^^^^^^^^
  File "C:/msys64/home/William/Projects/python/pdm/src/pdm/models/python.py", line 53, in version
    return self._py_ver.version
           ^^^^^^^^^^^^^^^^^^^^
  File "C:/msys64/mingw64/lib/python3.11/site-packages/findpython/python.py", line 81, in version
    self._version = self._get_version()
                    ^^^^^^^^^^^^^^^^^^^
  File "C:/msys64/mingw64/lib/python3.11/site-packages/findpython/python.py", line 180, in _get_version
    version = _run_script(
              ^^^^^^^^^^^^
  File "C:/msys64/mingw64/lib/python3.11/site-packages/findpython/python.py", line 23, in _run_script
    return subprocess.run(
           ^^^^^^^^^^^^^^^
  File "C:/msys64/mingw64/lib/python3.11/subprocess.py", line 548, in run
    with Popen(*popenargs, **kwargs) as process:
         ^^^^^^^^^^^^^^^^^^^^^^^^^^^
  File "C:/msys64/mingw64/lib/python3.11/subprocess.py", line 1026, in __init__
    self._execute_child(args, executable, preexec_fn, close_fds,
  File "C:/msys64/mingw64/lib/python3.11/subprocess.py", line 1538, in _execute_child
    hp, ht, pid, tid = _winapi.CreateProcess(executable, args,
                       ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
FileNotFoundError: [WinError 2] The system cannot find the file specified

This patch looks for the virtualenv python binary in an additional location (under bin) before falling back to the conda python binary.

[codecov]

Codecov Report

Patch coverage is 50.00% of modified lines.

Files Changed	Coverage
src/pdm/models/venv.py	50.00%

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