git lfs issues

Even with git subrepo it seems that we can sometimes have issues with git lfs.

```
❯ ctf challenge add ....
git-subrepo: Command failed: 'git read-tree --prefix=...... -u 4dda58e74a75de46c39............'.
Downloading .......file....... (.........)
Error downloading object: .......file....... (123456): Smudge error: Error downloading .......file....... (.......hash.......): [.......hash....... ] Object does not exist on the server: [404] Object does not exist on the server

Errors logged to 'filename.log'.
Use `git lfs logs last` to view the log.
error: external filter 'git-lfs filter-process' failed
fatal: .......file.......: smudge filter lfs failed
Could not add the challenge repository. Please check git error messages above.
````

You can work around this with the following and then manually pulling the lfs files as referenced in the lower links

```
export GIT_LFS_SKIP_SMUDGE=1
```

https://github.com/ingydotnet/git-subrepo/issues/152
https://github.com/ingydotnet/git-subrepo/issues/152#issuecomment-591540592

We should still try to have a better flow for these large files though.