gh-89427: Provide the original prompt value for VIRTUAL_ENV_PROMPT (GH-106726)

This improves the implementation in gh-106643.

Previously, venv passed "(<prompt>) " to the activation scripts, but we want
to provide the original value so that users can inspect it in the
$VIRTUAL_ENV_PROMPT env var.

Note: Lib/venv/scripts/common/Activate.ps1 surrounded the prompt value with
parens a second time, so no change was necessary in that file.