Error when %TEMP% is a junction to a dir on a letter-less drive

[grv87]

Environment Versions

1. OS Type: Windows 10 x64
2. Python version: $ python -V: 3.10.0, 3.9.6, 3.8.2
3. pip version: 21.3
4. pip-tools version: $ pip-compile --version: no

Steps to replicate

1. Create a volume and don't assign letter to it
2. Create dir on this volume
3. Create a junction on this dir:
   [ln](https://schinagl.priv.at/nt/ln/ln.html) --junction junction ‪\\?\Volume{<new volume uuid>}\dir
4. set TEMP=%cd%\junction
5. Install some package: pip3 install CrossMap

Expected result

Successful install/reinstall

Actual result

Traceback (most recent call last):
  File "<python_install_dir>\lib\site-packages\pip\_internal\cli\base_command.py", line 164, in exc_logging_wrapper
    status = run_func(*args)
  File "<python_install_dir>\lib\site-packages\pip\_internal\cli\req_command.py", line 204, in wrapper
    return func(self, options, args)
  File "<python_install_dir>\lib\site-packages\pip\_internal\commands\install.py", line 294, in run
    wheel_cache = WheelCache(options.cache_dir, options.format_control)
  File "<python_install_dir>\lib\site-packages\pip\_internal\cache.py", line 219, in __init__
    self._ephem_cache = EphemWheelCache(format_control)
  File "<python_install_dir>\lib\site-packages\pip\_internal\cache.py", line 196, in __init__
    super().__init__(self._temp_dir.path, format_control)
  File "<python_install_dir>\lib\site-packages\pip\_internal\cache.py", line 121, in __init__
    super().__init__(cache_dir, format_control, {"binary"})
  File "<python_install_dir>\lib\site-packages\pip\_internal\cache.py", line 44, in __init__
    assert not cache_dir or os.path.isabs(cache_dir)
AssertionError

Error looks similar to #1061, but in different place.

[grv87]

I reported it in the wrong repo. Correct report: pypa/pip#10597