maintainers/scripts/haskell: don't redundantly add C++ Nix to env

Since the shebang calls nix-shell, we can safely assume that Nix (Lix,
C++ Nix) is installed. Our scripts should support a wide enough range of
Nix versions so that using the “impure” version of the tool is not a
problem.

This works around #400784. My theory is that the Nix frontend commands
no longer work with older versions of the Nix daemon nor the Lix daemon
in our workloads.

maintainers/scripts/haskell/update-hackage.sh

@@ -1,5 +1,5 @@
 #! /usr/bin/env nix-shell
-#! nix-shell -i bash -p nix curl jq git gnused -I nixpkgs=.
+#! nix-shell -i bash -p curl jq git gnused -I nixpkgs=.
 
 # See regenerate-hackage-packages.sh for details on the purpose of this script.