Override toxworkdir with --workdir.

[q0w]

Thanks for contribution

Closes #2654
Please, make sure you address all the checklists (for details on how see
development documentation)!

• ran the linter to address style issues (tox -e fix)
• wrote descriptive pull request text
• ensured there are test(s) validating the fix
• added news fragment in docs/changelog folder
• updated/extended the documentation

[gaborbernat]

Can we use the primary key for this instead?

[q0w]

Can we use the primary key for this instead?

how?

[q0w]

@gaborbernat should work_dir end with .tox?

[gaborbernat]

Only by default, if the user sets a dir we should use that.

[markferry]

I'm not sure how yet but bf87bea reliably breaks our pyscaffoldext-custom-extension using Jenkins on Windows (python 3.8).

I bisected tox all the way back between 4.0.16 (which was good) and latest.
No code changed and no python deps changed apart from tox.

tmpfolder = WindowsPath('C:/Users/zSvcbuildmaster01/AppData/Local/Temp/pytest-of-unknown/pytest-99/test_generated_extension0/tmpqbuyf27f')

...
Error while running command:
['D:\\\\jenkins\\\\workspace\\\\pyscaffoldext-mything_master\\\\.tox\\\\test-windows\\\\Scripts\\\\python.EXE', '-m', 'tox']
{'stderr': -2, 'universal_newlines': True}

test-windows: internal error
Traceback (most recent call last):
  File "D:\\jenkins\\workspace\\pyscaffoldext-mything_master\\.tox\\test-windows\\lib\\site-packages\\tox\\session\\cmd\\run\\single.py", line 45, in _evaluate
    tox_env.setup()
  File "D:\\jenkins\\workspace\\pyscaffoldext-mything_master\\.tox\\test-windows\\lib\\site-packages\\tox\\tox_env\\api.py", line 248, in setup
    self._setup_env()
  File "D:\\jenkins\\workspace\\pyscaffoldext-mything_master\\.tox\\test-windows\\lib\\site-packages\\tox\\tox_env\\python\\runner.py", line 106, in _setup_env
    super()._setup_env()
  File "D:\\jenkins\\workspace\\pyscaffoldext-mything_master\\.tox\\test-windows\\lib\\site-packages\\tox\\tox_env\\python\\api.py", line 198, in _setup_env
    self.ensure_python_env()
  File "D:\\jenkins\\workspace\\pyscaffoldext-mything_master\\.tox\\test-windows\\lib\\site-packages\\tox\\tox_env\\python\\api.py", line 205, in ensure_python_env
    self.create_python_env()
  File "D:\\jenkins\\workspace\\pyscaffoldext-mything_master\\.tox\\test-windows\\lib\\site-packages\\tox\\tox_env\\python\\virtual_env\\api.py", line 129, in create_python_env
    self.session.run()
  File "D:\\jenkins\\workspace\\pyscaffoldext-mything_master\\.tox\\test-windows\\lib\\site-packages\\virtualenv\\run\\session.py", line 42, in run
    self._create()
  File "D:\\jenkins\\workspace\\pyscaffoldext-mything_master\\.tox\\test-windows\\lib\\site-packages\\virtualenv\\run\\session.py", line 49, in _create
    self.creator.run()
  File "D:\\jenkins\\workspace\\pyscaffoldext-mything_master\\.tox\\test-windows\\lib\\site-packages\\virtualenv\\create\\creator.py", line 153, in run
    self.create()
  File "D:\\jenkins\\workspace\\pyscaffoldext-mything_master\\.tox\\test-windows\\lib\\site-packages\\virtualenv\\create\\via_global_ref\\builtin\\via_global_self_do.py", line 89, in create
    src.run(self, self.symlinks)
  File "D:\\jenkins\\workspace\\pyscaffoldext-mything_master\\.tox\\test-windows\\lib\\site-packages\\virtualenv\\create\\via_global_ref\\builtin\\ref.py", line 151, in run
    method(self.src, dest)
  File "D:\\jenkins\\workspace\\pyscaffoldext-mything_master\\.tox\\test-windows\\lib\\site-packages\\virtualenv\\util\\path\\_sync.py", line 33, in copy
    ensure_safe_to_do(src, dest)
  File "D:\\jenkins\\workspace\\pyscaffoldext-mything_master\\.tox\\test-windows\\lib\\site-packages\\virtualenv\\util\\path\\_sync.py", line 23, in ensure_safe_to_do
    dest.unlink()
  File "C:\\Python38\\lib\\pathlib.py", line 1324, in unlink
    self._accessor.unlink(self)
PermissionError: [WinError 5] Access is denied: 'D:\\\\jenkins\\\\workspace\\\\pyscaffoldext-mything_master\\\\.tox\\\\test-windows\\\\Scripts\\\\python.exe'
test-windows: FAIL ! in 0.3 seconds