haskell-generic-builder: fix buildTarget specification if isLibrary

If we don't pass buildTarget to ./Setup copy and buildTarget is not
empty it will try installing targets that don't exist and thus fail.

pkgs/development/haskell-modules/generic-builder.nix

@@ -481,7 +481,7 @@ stdenv.mkDerivation ({
       # ^^ if the project is not a library, and we have a build target, then use "copy" to install
       # just the target specified; "install" will error here, since not all targets have been built.
     else ''
-      ${setupCommand} copy
+      ${setupCommand} copy ${buildTarget}
       local packageConfDir="$out/lib/${ghc.name}/package.conf.d"
       local packageConfFile="$packageConfDir/${pname}-${version}.conf"
       mkdir -p "$packageConfDir"