CI fuzzing corpus "resource not accessible by integration" for PRs from external clones

In #815 the fuzzing in CI failed with:
```
Error: Resource not accessible by integration
Error: Process completed with exit code 1.
```

I think that's just because I have CI accumulating the corpus for fuzzing runs cumulatively between PRs, and I guess there's some kind of permissions thing happening with PRs from other repos. I actually thought I addressed that already, but apparently not.