Update static Nix build

[Gabriella439]

This is a revamp of our use of
fully static Haskell executables.
The main changes are:

• We can now use a newer revision of Nixpkgs

  The current revision we're using corresponds to Nixpkgs 20.03

• We can now more easily update Nixpkgs

  The logic in this repository more closely tracks how static-haskell-nix
  works, so whenever that repository updates Nixpkgs we can pick up the change
  here, too.

  Note that we cannot currently use Nixpkgs master due to
  this issue, but once that is
  fixed when we can track Nixpkgs master more freely.

• We no longer need to keep track of separate Nixpkgs revisions for
  static vs. non-static builds

  The two builds can also share dependencies, too. Only the top-level
  derivations differ now.

The reason that this doesn't use static-haskell-nix directly is because of
cabal2nix evaluation failures,
so this change works by lifting just the minimum logic that we need from
the static-haskell-nix repository until we can finally use that
repository (or Nixpkgs directly when it's upstreamed).

[Gabriella439]

I'm going to merge this as is (since kubernetes-dhall-docs doesn't block CI) and create a separate pull request to fix the kubernetes-dhall-docs build