[3.12] gh-105699: Fix a Crasher Related to a Deprecated Global Variable (gh-106923) (#106964)

gh-105699: Fix a Crasher Related to a Deprecated Global Variable (gh-106923)

There was a slight race in _Py_ClearFileSystemEncoding() (when called from _Py_SetFileSystemEncoding()), between freeing the value and setting the variable to NULL, which occasionally caused crashes when multiple isolated interpreters were used.  (Notably, I saw at least 10 different, seemingly unrelated spooky-action-at-a-distance, ways this crashed. Yay, free threading!)  We avoid the problem by only setting the global variables with the main interpreter (i.e. runtime init).
(cherry picked from commit 0ba07b2108d4763273f3fb85544dde34c5acd40a)

Co-authored-by: Eric Snow <ericsnowcurrently@gmail.com>