Basically you want it to call fchdir() instead of chdir() when passed a fd (integer) instead of a string/Path-like.  That makes sense and should be a reasonably straight forward set of changes to _posixsubprocess.c.

(A way to convert a fd into a Path-like object would _not_ work as that'd reintroduce the TOCTOU on the directory - that'd be a pathlib feature request anyways, not a subprocess one)