gh-99578: Fix refleak in _imp.create_builtin() (GH-99642)

Fix a reference bug in _imp.create_builtin() after the creation of
the first sub-interpreter for modules "builtins" and "sys".
(cherry picked from commit cb2ef8b2acbb231c207207d3375b2f8b0077a6ee)

Co-authored-by: Victor Stinner <vstinner@python.org>