[asan] disable MallocNanoZone for no fd test on darwin

On Darwin, MallocNanoZone may log after execv, which messes up this test.
Disable MallocNanoZone for this test since we don't use it anyway with asan.

This environment variable should only affect Darwin and not change behavior on other platforms.

rdar://74992832

Reviewed By: delcypher

Differential Revision: https://reviews.llvm.org/D98735