Rollup merge of #58438 - cuviper:posix_spawn_file_actions_addchdir_np, r=alexcrichton

Use posix_spawn_file_actions_addchdir_np when possible

This is a non-POSIX extension implemented in Solaris and in glibc 2.29.
With this we can still use `posix_spawn()` when `Command::current_dir()`
has been set, otherwise we fallback to `fork(); chdir(); exec()`.