Motivation for this change

The latest snap on hackage is buildable from nixpkgs, since it bumped dependencies for ghc-8.8.2.

Apologies for not being familiar with the nixpkgs haskell process (e.g. - I'm not sure if inclusion on the broken list is automated, making this PR unnecessary).

cc @cdepillabout (Thanks for your haskell-updates call-to-action!)

If this goes through, I can test the many snap-* dependencies which were probably only marked broken because snap itself was marked broken, and unbreak those.

Things done

• Tested using sandboxing (nix.useSandbox on NixOS, or option sandbox in nix.conf on non-NixOS linux)
• Built on platform(s)
  • NixOS
  • macOS
  • other Linux distributions
• Tested via one or more NixOS test(s) if existing and applicable for the change (look inside nixos/tests)
• Tested compilation of all pkgs that depend on this change using nix-shell -p nixpkgs-review --run "nixpkgs-review wip"
• Tested execution of all binary files (usually in ./result/bin/)
• Determined the impact on package closure size (by running nix path-info -S before and after)
• Ensured that relevant documentation is up to date
• Fits CONTRIBUTING.md.