Relax conditional checking if a symlink already exists

If a symlink already exists AND it's pointing inside the `~/.fresh/build` directory we can clobber it.

This handles errors caused by a change to how we handle basenames (5e94c91c88b64e66cacc4fe047bb79051d0e0e76).

---

Example error:

```
Error: /Users/odin/.vim/colors/bclear_custom.vim already exists (pointing to /Users/odin/.fresh/build/bclear_custom.vim)
~/.freshrc:40: fresh vim/colors/bclear_custom.vim --file=~/.vim/colors/bclear_custom.vim

You may need to run `fresh update` if you're adding a new line,
or the file you're referencing may have moved or been deleted.
```