virtualenv: Pip seeding with download omits wheel digest verification

virtualenv is a tool for creating isolated virtual python environments. Prior to 21.14.9, PipInvoke.get_pip_install_cmd() handles --seeder pip --download by calling get_wheel(download=False) and then allowing pip to consult the configured index, bypassing virtualenv's download_wheel() and verify_wheel_digest() path. When virtualenv's default-index PyPI digest comparison remains enabled, an index candidate matching the bundled package and version can therefore be selected without comparison to PyPI SHA-256 metadata. Explicit custom-index settings already opt out of that comparison and remain outside this verification-bypass claim; the demonstrated configuration used a non-PyPI index while leaving the index environment variables unset so the comparison policy remained active. This condition does not independently defeat TLS or establish control of the index response. This issue is fixed in version 21.14.9.