[3.13] gh-122941: Fix test_launcher sporadic failures via py.ini isolation (GH-145090)

Adds _PYLAUNCHER_INIDIR as a private variable since the launcher is deprecated and not getting new features.
(cherry picked from commit 6cdbd7bc5d4ee63459d03a944477ea8671a05198)

Co-authored-by: Itamar Oren <itamarost@gmail.com>