No way to configure corporate pip index URL for environment creation / package install UI on Windows

@edvilme がすでに取り組んでいます。

2026年7月1日 から。

### Environment
- OS: Windows 11 Enterprise
- VS Code: 1.104.x
- Python Environments extension: latest (1.20.x)
- Python: 3.11 / 3.12
- Package manager: pip / uv
- Network: corporate network with private PyPI mirror (`--index-url https://pypi.corp/simple`) and strict `--no-default-index` policy (public pypi.org blocked)

### Problem
The extension's "Create Environment" and "Install Package" UI invokes `pip install` / `uv pip install` without respecting a configured `--index-url` or `--extra-index-url`. On corporate machines where `pypi.org` is blocked:

1. Creating a new venv via the extension's GUI picker succeeds, but any package the extension auto-installs (e.g., `pip` upgrade, `wheel`) fails silently.
2. The "Packages" panel's "Install" button runs `pip install ` against `pypi.org` — always fails.
3. There is no `python-envs.pip.indexUrl` or equivalent setting to redirect to the internal mirror.

### Expected
Expose a setting:
```jsonc
"python-envs.packageManager.extraArgs": ["--index-url", "https://pypi.corp/simple", "--trusted-host", "pypi.corp"]
```
that is appended to every `pip install` / `uv pip install` invocation the extension makes on behalf of the user.

Alternatively: the extension should inherit `pip.ini` / `pip.conf` if already set in the system (pip already supports this), and the README should document that `%APPDATA%\pip\pip.ini` is the right place to set `[global] index-url` for the extension to pick up.

### Workaround
Setting `[global] index-url` in `%APPDATA%\pip\pip.ini` works for terminal invocations but the extension appears to spawn pip in a way that may not pick up user-level pip.ini on Windows (path resolution differs for subprocess vs terminal shell).