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

Description

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

The error looks similar to jazzband/pip-tools#1061, but in pip itself.

Expected behavior

Successful install/reinstall

pip version

21.3

Python version

3.10.0, 3.9.6, 3.8.2

OS

Windows 10 x64

How to Reproduce

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

Output

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

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