GHC built with musl via nix

Contained here is a build of GHC with musl via nix.

Essentially it allows you to run the command to build a small example program as described in NixOS#37598 (comment), but importing the below nix closure I've pre-compiled allows you to skip 2 compilations of GHC.

curl https://github.com/nh2/nixpkgs/releases/download/ghc-cross-musl-works/ghc-musl.closure.xz | xz -d | nix-store --import

Then run the command from NixOS#37598 (comment) again to build the example program.

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The GHC built this way is the one built by the command in the linked comment (branch name replaced by SHA at that time):

NIX_PATH=nixpkgs=https://github.com/dtzWill/nixpkgs/archive/7048fc71e325c69ddfa62309c0b661b430774eac.tar.gz nix-build '<nixpkgs>' --arg crossSystem '{config="x86_64-unknown-linux-musl";}' -A haskell.packages.ghc841.hello 

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Note it's a bit confusing that this is filed under the tag ghc-cross-musl-works when in fact it uses branch feature/ghc-cross-musl. My mistake; whatever, it's just naming.