bpo-40448: ensurepip: Do not use cache (GH-19812)

ensurepip optionally installs or upgrades 'pip' and 'setuptools' using
the version of those modules bundled with Python.  The internal PIP
installation routine by default temporarily uses its cache, if it
exists.  This is undesirable as Python builds and installations may be
independent of the user running the build, whilst PIP cache location
is dependent on the user's environment and outside of the build
environment.

At the same time, there's no value in using the cache while installing
bundled modules.

This change disables PIP caching when used in ensurepip.
(cherry picked from commit 4a3a682b12f93a03888e8b59f439bc5fe30d6055)

Co-authored-by: Krzysztof Konopko <kkonopko@users.noreply.github.com>

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