Fix Problems with Template Haskell on NixOS

[jkachmar]

On NixOS there are a few problems with trying to use the statically linked Haskell Language Server binary with Template Haskell invocations.

cf haskell/haskell-language-server#122 (comment) for details

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Some links that might help:

• https://nixos.wiki/wiki/Packaging/Binaries
  • skip patching since these are static binaries on NixOS
• https://nixos.org/manual/nixpkgs/stable/#sec-fhs-environments
  • buildFHSUserEnv might be the best call here
  • https://discourse.nixos.org/t/runtime-alternative-to-patchelf-set-interpreter/3539/20
    • some details about setting the interpreter

[jkachmar]

The project will probably have to be refactored a bit.

The macOS expressions are fine (because macOS doesn't support static linking), but the NixOS expressions are going to end up being relatively different if the solution ends up relying on buildFHSUserEnv.

Probably the best way to do this is have a sub-directory with macos.nix and nixos.nix, and then default.nix can use the isLinux/isDarwin logic to select the appropriate expression.

Maybe also move some logic out into a common module for selecting GHC support versions (etc.) that are common to both sub-expressions? idk.