Honor `$TEMP` and friends when generating anonymous handles (#43515)
On Darwin, when generating anonymous handles, we hardcoded `/tmp` as the
beginning path, but that's bad behavior if the user has explicitly
requested (via setting `$TEMP` or `$TEMPDIR`, etc...) that we not do so.
Let's use `uv_os_tmpdir()` to put things in the correct place as much as
we can, defaulting to `/tmp` if anything goes wrong inside of libuv.