Fix running build.sh from other directories

When running `build.sh` from any directory other than the root directory of `ycmd` or `YouCompleteMe`, it prints the following output regardless of whether the `third_party` is available, because it's looking for `third_party` in `pwd`.

```
find: third_party/*: No such file or directory
Some folders in ./third_party are empty; you probably forgot to run:

	git submodule update --init --recursive
```