[3.12] gh-109853: Fix sys.path[0] For Subinterpreters (gh-109994) (gh-110701)

This change makes sure sys.path[0] is set properly for subinterpreters.  Before, it wasn't getting set at all.

This change does not address the broader concerns from gh-109853.

(cherry-picked from commit a040a32ea2f13f16172394d3e3e3f80f47f25a68)