ci: Switch to GHC 8.4.4

We previously used the haskell:8.4 docker image, which provides the
latest version of GHC 8.4. Unfortunately, when GHC was upgraded from
8.4.3 to 8.4.4 in the docker image, stack broke because it expected
8.4.3.

To prevent this from happening again, we now use the 8.4.4 docker
image explicitly.

This means that CI is now 8.4.4 instead of 8.4.3.