ci: macos: Remove use of -Dsegsize_blocks=6

ci: macos: Remove use of -Dsegsize_blocks=6

The option 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.

d3b111e3205 used a small segment size for two tasks, one with autoconf, one
with meson. In hindsight that is a bit overkill, it's unlikely that the option
would silently break. Thus don't move the -Dsegsize_blocks=6, just remove
it. I did however change the autoconf test to use 6 instead of 8 blocks, as
long as we allow it, a non-power-of-two test seems like a good idea.

While at it, add a comment explaining why we use a small segment size for CI.

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 d3b111e3205 introduced the use of -Dsegsize_blocks=6