Reintroduce cabal-install in flake.nix (#3637)

PR #3555 removes `cabal-install` in order to "make CI green" but
that makes the project unbuildable using nix. Since the PR mentions not
being sure of the details and more people at ZuriHac are having trouble
with this, I'm suggesting reintroducing it.

Author: cgeorgii

flake.nix

@@ -237,6 +237,7 @@
           buildInputs = [
             # our compiling toolchain
             hpkgs.ghc
+            hpkgs.cabal-install
             # @guibou: I'm not sure hie-bios is needed
             # pkgs.haskellPackages.hie-bios
             # Dependencies needed to build some parts of hackage