drop fork and always use the posix_spawn family of functions

[glyph]

As mentioned on #12071 , os.fork is now throwing a deprecation warning.

Better than falling back to os.fork for features that are not yet supported by posix_spawn would be falling back to an executable stub that can perform all the post-exec work.

The reason we don't do this right now is that we don't want to pay the exec-Python penalty twice, but a minimal Rust stub could mitigate this significantly; we would still have only one call to posix_spawn, the executable in question would do its minimal bootstrap work, then exec.

We don't need to worry about adding a new hard binary dependency here either, because the slow Python-only fallback here would also be extremely short.