make cssmin operate on the generated file

Both sass and less put their results in dirs.tmp.  That's also where the copy:finalize task expects to find the final result.  Therefore, make cssmin operate on the main.css file in-place.

Without this change, cssmin always seems to complain:

```
Running "cssmin:compress" (cssmin) task
>> Destination not written because minified CSS was empty.
```