this mostly requires plumbing to accept an int as the cwd and plumb that through to the between fork and exec code to call `fchdir(cwd_fd)` on the `int` instead of chdir(cwd) on the `char*`.

the Modules/_posixsubprocess.c internals are a bit of a mess today with a bazillion parameters passed to internal functions which makes this a pain... I want to refactor things to use a struct and not need that, but adding this feature is doable regardless.