test: skip POSIX-venv wheel-build tests on Windows (tox-dev#4078)

## What

`test_build_wheel_in_non_base_pkg_env`,
`test_build_wheel_in_free_threaded_pkg_env`,
and `test_build_wheel_in_debug_pkg_env` (in
`tests/tox_env/python/test_python_api.py`)
error in setup on Windows without elevation or Developer Mode:

```
OSError: [WinError 1314] A required privilege is not held by the client
```

Each mocks a virtualenv via `_fake_session`, which builds a POSIX layout
by creating a
`bin/python` symlink to `sys.executable`. `os.symlink` needs a privilege
an ordinary
Windows user doesn't have, and the `bin/python` layout is POSIX-only
regardless (Windows
venvs use `Scripts\python.exe`). CI stays green because the
`windows-latest` runner can
create symlinks, so this only bites an unprivileged Windows
contributor's local run.

## Fix

Guard the three tests with `@pytest.mark.skipif(sys.platform == "win32",
...)`, matching
the convention already used throughout the suite (e.g.
`tests/execute/local_subprocess/`).

## Note

Verified with `py_compile` and by inspection. If you'd rather keep
Windows coverage by
making the mock cross-platform (`Scripts/python.exe` on Windows, no
symlink), happy to do
that instead — the skip is the minimal, convention-matching option.

🤖 Generated with [Claude Code](https://claude.com/claude-code)

Co-authored-by: Claude Opus 4.8 <noreply@anthropic.com>

Author: iam-kira

tests/tox_env/python/test_python_api.py

@@ -80,6 +80,7 @@ def test_validate_base_python_conflicting_factors_no_ignore() -> None:
         Python._validate_base_python("unit-py3.10-2.16", ["python3"], ignore_base_python_conflict=False)  # ruff:ignore[private-member-access]
 
 
+@pytest.mark.skipif(sys.platform == "win32", reason="mocks a POSIX venv layout (bin/python symlink)")
 def test_build_wheel_in_non_base_pkg_env(
     tox_project: ToxProjectCreator,
     patch_prev_py: PatchPrevPy,
@@ -112,6 +113,7 @@ def _fake_session(env_dir: list[str], **_: object) -> MagicMock:
     ]
 
 
+@pytest.mark.skipif(sys.platform == "win32", reason="mocks a POSIX venv layout (bin/python symlink)")
 def test_build_wheel_in_free_threaded_pkg_env(
     tox_project: ToxProjectCreator,
     patch_prev_py: PatchPrevPy,
@@ -144,6 +146,7 @@ def _fake_session(env_dir: list[str], **_: object) -> MagicMock:
     ]
 
 
+@pytest.mark.skipif(sys.platform == "win32", reason="mocks a POSIX venv layout (bin/python symlink)")
 def test_build_wheel_in_debug_pkg_env(
     tox_project: ToxProjectCreator,
     patch_prev_py: PatchPrevPy,