haskell-packages: properly set postPatch

The existing code overrode the postPatch, instead of combining the
jailbreak commands with the user supplied postPatch.

pkgs/development/compilers/ghcjs/default.nix

@@ -52,7 +52,7 @@ mkDerivation (rec {
   };
   isLibrary = true;
   isExecutable = true;
-  jailbreak = false; # manually jailbreak, until postPatch in mkDerivation is fixed
+  jailbreak = true;
   doHaddock = false;
   doCheck = false;
   buildDepends = [
@@ -72,9 +72,6 @@ mkDerivation (rec {
   ];
   patches = [ ./ghcjs.patch ];
   postPatch = ''
-    echo "Run jailbreak-cabal to lift version restrictions on build inputs."
-    ${jailbreak-cabal}/bin/jailbreak-cabal ${pname}.cabal
-
     substituteInPlace Setup.hs \
       --replace "/usr/bin/env" "${coreutils}/bin/env"