Include cabal-install and ghc in shell environment

ci/default.nix

@@ -142,6 +142,6 @@ let
 in
   mkShell {
     name = "head-hackage-build-env";
-    buildInputs = [ build-repo ];
+    buildInputs = [ build-repo haskellPackagesWithCabal.cabal-install haskellPackages.ghc ];
     cabalDepsSrc = buildDepsFragment;
   }