print env vars when running nix-build commands.

src/Argonix.hs

@@ -324,6 +324,8 @@ wrap nixCommand target sa@ArgsCommon {..} = sh $ do
   when (verbosity == Verbose) $ liftIO $ print sa
   _ <- setupSystem sa
   printCommand $ pack nixc <> " " <> pack (unwords arglist)
+  void $ shell "echo \"TMPDIR=$TMPDIR\"" empty
+  void $ shell "echo \"XDG_CACHE_HOME=$XDG_CACHE_HOME\"" empty
   liftIO (executeFile nixc True arglist Nothing)
  where
   nixc = toCommand nixCommand