ci: macos: Remove use of -DRANDOMIZE_ALLOCATED_MEMORY

ci: macos: Remove use of -DRANDOMIZE_ALLOCATED_MEMORY

RANDOMIZE_ALLOCATED_MEMORY causes a measurable slowdown. Macos is, by far, the
most expensive platform for CI, therefore it doesn't make sense to run such a
test there.

Ubsan and asan on linux should detect most of the the cases of uninitialized
memory, so it doesn't really seem worth using -DRANDOMIZE_ALLOCATED_MEMORY in
another instance type.

Author: Andres Freund <andres@anarazel.de>
Reviewed-by: Heikki Linnakangas <hlinnaka@iki.fi>
Discussion: https://postgr.es/m/20230808021541.7lbzdefvma7qmn3w@awork3.anarazel.de
Backpatch: 16-, where 89d16b635 added the use of -DRANDOMIZE_ALLOCATED_MEMORY