Add expression for building haskell-language-server, based on a vendored
version of https://github.com/korayal/hls-nix with hashes updated to
work with our nixpkgs version and ghc 8.8.3. Also add that to CI
builds, so whitby will build it for me (thanks whitby).
Change-Id: I373f8a7cb67974b8aa043b116436c074591b8d57
Reviewed-on: https://cl.tvl.fyi/c/depot/+/897
Tested-by: BuildkiteCI
Reviewed-by: tazjin <mail@tazj.in>
Reviewed-by: BuildkiteCI
