haskellPackages.vaultenv: run hpack in prePatch

This matches cabal2nix generated expressions. We assume that the cabal
file exists after the user supplied prePatch.

pkgs/development/tools/haskell/vaultenv/default.nix

@@ -46,6 +46,7 @@ mkDerivation rec {
   prePatch = ''
     substituteInPlace package.yaml \
         --replace -Werror ""
+    hpack
   '';
 
   isLibrary = false;
@@ -82,7 +83,6 @@ mkDerivation rec {
     hspec-expectations
     quickcheck-instances
   ];
-  preConfigure = "hpack";
   homepage = "https://github.com/channable/vaultenv#readme";
   description = "Runs processes with secrets from HashiCorp Vault";
   license = lib.licenses.bsd3;