The NonStop platform does not have `mkdtemp()` available, which we rely
on in `build_sandbox_path()`. Fix this issue by using `mktemp()` and
`mkdir()` instead on this platform.
This has been cherry-picked from the upstream pull request at [1].
[1]: https://github.com/clar-test/clar/pull/96
Signed-off-by: Patrick Steinhardt <ps@pks.im>
Signed-off-by: Junio C Hamano <gitster@pobox.com>