ci: Change macOS builds from Intel to ARM.
Cirrus is about to shut down its macOS-on-Intel support, so it's time to
move our CI testing over to ARM instances. The Homebrew package manager
changed its default installation prefix for the new architecture, so a
couple of tests need tweaks to find binaries.
Back-patch to 15, where in-tree CI began.
Author: Justin Pryzby <pryzby@telsasoft.com>
Discussion: https://postgr.es/m/
20221122225744.GF11463%40telsasoft.com