too many spack runs in CI

[edwardhartnett]

I think checking each combination of options is too much. THere are dozens of spack CI runs, which really slow down testing and also burn up a lot of electricity. How can we reduce them to about 5 or 6 spack builds at most?

[AlexanderRichert-NOAA]

445 reduces it to two, and we can always add other combinations later as needed.