Match GHC build environments in CI?

@mpickering and @wz1000 suggested to build the HLS binaries on the same environment that was used to build the corresponding GHC.

I think for windows, we don't have to worry about it.

For darwin, we do have to worry. I'm not sure how you propose to achieve this. I myself won't maintain any nix code, so that's for someone else to implement.

For linux, we could just reuse the docker images. We'll have to check with @bgamari whether they're cleaned up or not for old GHCs, such as 8.10.7.

I'm not sure this will be without problems. I'll have to think more about it.