Rework Darwin jobs

* Use `arch` explicitly in both jobs. I don't know why x86_64 is the
  default, and I don't like it.

* Stop using brew. This means a toolchain needs to already be installed
  where CI is run. This is the case today, although it's a bit fragile
  and I'll probably revisit it.

* Use a matrix build to clean up the yml.

* Use GHC 9.8.2 to work around GHC#24050