Test ghc-9.4 in CI

[gelisam]

The GitHub action haskell/actions/setup does not support ghc-9.4.1 yet, so we'll have to merge #152 without a regression test. This issue exists to remind me to add that regression test once either stack or haskell/actions/setup supports this version.

[christiaanb]

We have GitHub actions support for some of our 9.4.1 upgrades of some of our code. See e.g. https://github.com/clash-lang/ghc-typelits-knownnat/runs/7486517906?check_suite_focus=true

The GitHub actions configuration was mostly autogenerated using: https://github.com/haskell-CI/haskell-ci

I wouldn’t mind setting up something similar as part of #152

[byorgey]

Looks like haskell/actions/setup now supports GHC 9.4.

[gelisam]

Fixed by #152