Migrate nix config to unbreak darwin CI

Current nix config is here: https://gitlab.haskell.org/haskell/ghcup-hs/-/blob/219cba5fc7f33814f0ba41b019628c085275614a/.gitlab/shell.nix

It was copy-pasted from ghc repo, because we share the same darwin M1 runners with nix cache. GHC however migrated the config to something new https://gitlab.haskell.org/ghc/ghc/-/tree/master/.gitlab/darwin ...breaking our CI in the process, because the cache is now constantly busted.

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Edit: we will not depend on nix anymore. So the CI config has to be migrated to brew (or something else). We can't expect ghcup users to install llvm via nix, so our own CI needs to reflect this reality.