gh-117505: Run ensurepip in isolated env in Windows installer (GH-118257)

ensurepip forks a subprocess to run pip itself, but that subprocess only inherits a -I isolated mode flag (see _run_pip() in Lib/ensurepip/__init__.py), not the "-E -s" flags that the installer has been using. This means that parts of ensurepip don't actually run in an isolated environment and can make incorrect decisions based on packages installed in the user site-packages.
(cherry picked from commit c9073eb1a99606df1efeb8959e9f11a8ebc23ae2)

Co-authored-by: Michael Vincent <377567+Vynce@users.noreply.github.com>