[lldb][lit] Add MallocNanoZone envvar to Darwin ASan builds (#88431)

When builds on Darwin are configured with ASan, running tests will emit
a warning about malloc's nano zone (`malloc: nano zone abandoned due to inability to reserve vm space`) that can interfere with the test
output and cause failures. Setting the environment variable
`MallocNanoZone` to 0 will remove this warning and allow the tests to
run as normal.