Cache ghc883 and ghc8101 again (without tests)

[hamishmack]

When ghc 8.8.4 and ghc 8.10.2 were added we stopped building the older
versions on ci. This changes adds them back but does not run all
the tests on them.

[michaelpj]

Worth writing down the policy here... what is it?

[michaelpj]

Actually, thinking about the policy, should we put something in the documentation (and maybe even in the readme) listing the GHC versions that get this extra level of support?

I think it's just two support levels:

1. We have an expression, and relevant bits are materialized, and we build the compiler itself and nix-tools (I think we do all of this for any version we have an expression for?)
2. The above, and it's tested.

Writing it down, I guess that's only one level as far as users are concerned.

[masaeedu]

@michaelpj I think that would be a good idea. Additionally, I'm not sure if it's too much trouble, but getting the CI to maintain some system of tags/branches which allows a user to get the latest version of haskell.nix for which cached builds are available with a particular compiler and nixpkgs would be very useful. At the moment the choice is between sitting on one fixed commit forever once you've finally got everything to work, or upgrading straight to master periodically (which can often cause breakages with a selected combination of nixpkgs, compiler and index-state).

[michaelpj]

I think realistically the only guarantee we offer is that master is built with whatever's in ci.nix.

[masaeedu]

@michaelpj The problem I'm running into is that it doesn't seem like we have the guarantee that everything in ci.nix is built by the time it's in master. I want some ref that I can put in my niv projects, such that when I run niv update I still get cache hits. master does not appear to be such a ref, because things seem to go into master before the corresponding stuff is built and stored in the cache.

[michaelpj]

Are you using the iohk hydra cache as well as the cachix one? (https://input-output-hk.github.io/haskell.nix/tutorials/getting-started/#setting-up-the-cachix-binary-cache)