[3.11] gh-104522: Fix OSError raised when run a subprocess (GH-114195) (GH-114243)

Only set filename to cwd if it was caused by failed chdir(cwd).

_fork_exec() now returns "noexec:chdir" for failed chdir(cwd).

(cherry picked from commit e2c097ebdee447ded1109f99a235e65aa3533bf8)

Co-authored-by: Robert O'Shea <PurityLake@users.noreply.github.com>